Allow iteration of TodoList items.

Thanks @mruwek for mentioning.

This shows more consistent behavior with other prompt applications.

This commit fixes issue #106.

Not all commands always need dependencies (pri, depri, ls -x), but all
administration is still built up every time a TodoList is instantiated.
Now, as soon as something dependency related is requested from the
TodoList, the dependency graph and caches are initialized.

A decorator is used to mark methods in TodoList that require an
initialized graph.

This change impacts the calculation of the average importance (based on
the todo's ancestors). Importance doesn't have a notion of TodoList (and
I would like to keep it that way), so the calculation needs access to
the parents somehow. This was done with a parents attribute stored with
each Todo item, carefully kept up to date by the TodoList. This is
cumbersome, instead, now a Todo item gets a parents() method that
obtains the parents from the TodoList that keeps the todo item.

todo_by_dep_id was quite inefficient due to the linear search, calling
Todo.tag_value() many times to fetch the dependency ID.

Instead, maintain a dictionary that maps a parent ID to a todo item so
it can be looked up more easily.

Call Todo::is_active() only once per match, this halves the execution
time (on average).

The except path is relatively often chosen, making these functions
(still) quite slow.

(Exceptions are slow in Python and they are what they are called:
exceptions, not common cases).

Store tags in a dictionary where the key is the tag name and the value
is a list of values for that tag.

This makes tag operations much quicker (especially TodoBase::has_tag()
and TodoBase::tag_values()). They made quite a dent in execution time
with heavy todo.txt files with many tags, due to the numerous list
comprehensions that took place.

Spotted by @mruwek.

arrow is not an optional dependency
Mark optional dependencies as such.

Remove an indirect dependency from the list.

The arrow library returns more fine grained humanized dates, taking time
of the day into account. topydo only works with dates, so 'just now'
actually means 'today'.

See also this arrow issue which requests more coarse grained human
dates:

https://github.com/crsmithdev/arrow/issues/96

Python handles one exception at a time (per thread), raising a new
exception while still handling another one will result in run-time
errors. By linking the second exception to the original one, Python will
handle it correctly.

This was encountered while developing the alternative column UI, which
bailed out on completing a todo item with (invalid) child todo items.

...which is Python 3.2 based.

When a filter with a creation date matches, don't process expression
with the remaining filters.

Addresses issue #86 which requested filters on completion date. The
following tags are recognized:

* create
* created
* creation
* complete
* completed
* completion

They are specific ordinal tag filters, except that these tags don't
exist in a todo item.

Filtering on completion date does not work when completed items are
automatically archived. Archived items are not read when invoking `ls`,
so the done.txt file should be read as the main file instead:

    topydo -t done.txt ls complete:today

Also in this commit, decouple ExpressionCommand from Filter class a bit.

The expression class shouldn't bother which expressions belong to which
filter classes, let the Filter module handle that.

Create placeholder for args in aliases

With this change there is now possible to forward args supplied on CLI
to desired placed in real command.

Example config:

[aliases]
star = tag {} star 1
unstar = tag {} star

Example usage:

`topydo star foo` will resolve to: `topydo tag foo star 1`
`topydo unstar foo` will resolve to: `topydo tag foo star`

Alternate for `top` command

Suppose the task:

Some item p:1

(there is no parent). Then the next time a new ID is selected, it would
return 1 because there is no id: tag with value 1. However, this will
result in an invalid dependency.

When a UID of 1aa is passed, the code path would try to convert it to a
linenumber (integer) when linenumbers are configured. That fails and
should be caught accordingly.

Uses new %P format and -N parameter.

`ls -N` will display one 'screen-ful' of lines (the number of lines in the
console, less 2).

%P will display the priority if one is given, or print a 'placeholder' space
if none is present.

Priority command: allow lowercase priorities to be input