Fixes issue #1.

Move logic to retrieve the Command instance to separate method.

Also implement a 'help' subcommand, that either prints general help, or
when followed by a subcommand, prints the subcommand's help.

Fixes #4.

* When using textual IDs, TodoListBase.number() wouldn't return an
  integer, making deletion fail (which expected an int).
* When updating the todo - UID map, erase it first. Otherwise deleted
* items will be still in there.

This fixes a bug that occurred with the combination of text IDs and
appending projects of the parent todo item.

When adding the project to a child todo, the text ID was not updated,
and the standard output would show an outdated text ID. The next
invocation of topydo would show the correct ID.

Now the ID is consistent between adding and listing.

Also add reference to where the code came from.

sorted() was run on a list of Todo objects, which may produce different
results. Instead, sort on the item's number (i.e. its index in the
TodoList).

This should fix the occasional failing of test_do_subtasks2.

sorted() was run on a list of Todo objects, which may produce different
results. Instead, sort on the item's number (i.e. its index in the
TodoList).

This should fix the occasional failing of test_do_subtasks2.

1. Tests for relative months and years are now executed against fixed
   test dates.
2. Add additional test for "special cases".
3. Add test for leap years.

Feature is not ready yet, but since it's a part of issue #2 I already went
ahead and pushed the first part.

- Increment months strictly: 01.01.2015 + 1m = 01.02.2015.
- If there are fewer days in target month, date is rounded down:
  31.10.2015 + 1m = 30.11.2015.
- Years are incremented as 12 months.
- Leap years are 366 days long: 01.03.2015 + 1y = 01.03.2016 (not
  29.02.2016 as it would have been if year was equal to 365d)

This can be used for ordinal tag filters, for instance for getting the
todo items with a date older than 6 months:

topydo ls 'due:<-6m'