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

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

This is done with catching TypeError and calling
topydo.cli.CLIApplicationsBase.usage.

Add UILoader.py as an entry_point and decide which UI to start from
there.

- do not match something like: '://example.org'
- match URLs at beginning of the line

-  METADATA (key:value pairs)
-  LINK (foo://bar.bla)

Unicode fixes for prompt branch

Enable history in prompt-mode

Otherwise it wil fail when <NUMBER> contains some unicode diacritics.

Also moved the utf8 utility to the superclass' module because more
command tests are using it now.

* Convert command line arguments from UTF-8 to Unicode right away. All
  Command classes should expect Unicode strings as arguments.
* Add test case for special characters for the 'ls' subcommand. Other
  commands are still likely to be broken.

python_2_unicode_compatible is broken in six-1.8 (which comes with
Debian) when imported in Python 3.

The idea is that we can focus on Unicode issues in this branch, getting
both the CLI and the Prompt mode properly in shape having Python 3
support.

Conflicts:
	test/AddCommandTest.py
	test/AppendCommandTest.py
	test/ArchiveCommandTest.py
	test/DepCommandTest.py
	test/DepriCommandTest.py
	test/DoCommandTest.py
	test/EditCommandTest.py
	test/IcalCommandTest.py
	test/ListCommandTest.py
	test/ListContextCommandTest.py
	test/ListProjectCommandTest.py
	test/PostponeCommandTest.py
	test/PriorityCommandTest.py
	test/SortCommandTest.py
	test/TagCommandTest.py
	topydo/cli/CLIApplicationBase.py

Addresses many (but most likely not all) Unicode issues as requested in
issue #21.