Fixes #43.

topydo was a tad slow, because all modules of all subcommands were
loaded with each invocation of topydo. Particularly, the ical subcommand
was quite slow because it loads modules from the icalendar package.

Add todos from file with '-f' flag.

1. Move _preprocess_input_todo adn _postprocess_input_todo inside
_add_todo.
2. Remove self.todo

Use unittest.mock wherever it's available

When obtaining a configuration object, it is now possible to pass
certain values to be overridden no matter what settings were made in the
configuration files.

This is used to let the -t and -d flags (location of todo.txt and
archive respectively) override anything that is mentioned in
configuration files.

This fixes a bug where the edit subcommand did not respect the -t flag,
because it was simply reading the location from the configuration file.
Now the configuration is overridden when -t is passed.

Thanks to Jacek for reporting the broken 'edit' command.

Fix unicode in ical.

Addresses Unicode issues under the umbrella of issue #21.

Two possible future improvements regarding editor selection:

* configure editor through configuration file
* set editor through a flag, e.g. topydo edit -p gedit

Since python 3.3 `mock` is part of standard library. Let's try to use
that before looking for third-party installation of it.

Import prompt-toolkit only if it's needed.

Also display more readable error message in case of absence of
prompt-toolkit.

This fixes #35

Fix EditCommand inheritance problems

Credit for idea goes to Bram Schoenmakers (@bram85).

Currently it contains only `_view` and `_filters` methods, which were
previously defined in ListCommand. Now both EditCommand and ListCommand
will inherit from it to solve problems inside EditCommand.

This fixes #28

1. Check output of `topydo edit -e`
2. Test if opening archive works via `topydo edit -d`

The trigger is that pylint doesn't support Python 3.2 anymore, and in
that case Travis CI should not run pylint.

This script takes an optional argument for the executable name of the
corresponding Python version (typically just 'python2' and 'python3').
When not given, 'python' is just run, whatever is in your $PATH. The
latter mode is used in Travis CI.

pylint doesn't understand that <date> - <timedelta> results in a date,
but thinks it results in a timedelta. Since timedelta has no isoformat()
method, it will complain.

* Moved one super() call outside the EditCommand class into a lambda
  function.
* Made EditCommand.execute() call its correct parent execute() (was:
  ListCommand). However, this would go wrong:

  EditCommand.execute() calls MultiCommand.execute()
  MultiCommand.execute() calls ListCommand.execute()
      Because EditCommand has multiple inheritance
  ListCommand.execute() is successful, but returns None
  MultiCommand.execute() takes that as failure, returns False
  EditCommand.execute() returns False

  This is fixed by adding return True to the execute() method of the
  parent classes.

This fixes all pylint errors.

Fix help in prompt-mode