added os.path.expanduser to filenames

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

Conflicts:
	topydo/lib/EditCommand.py

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

Conflicts:
	test/EditCommandTest.py

* 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.

$HOME is not set on every operating system (can you guess which one?).
This resulted in errors because a configuration file path was
constructed using an empty string.

So use a different approach that should work for most major OS's.

Because pypi is being an ass about re-uploading a new (unbroken)
archive.

Conflicts:
	test/DoCommandTest.py
	topydo/lib/DoCommand.py

With the invocation `tag 1` it would print invalid todo number while
todo 1 does exist. It should print the usage text in that case.

With the invocation `tag 1` it would print invalid todo number while
todo 1 does exist. It should print the usage text in that case.

Remove priorities from multiple todo items at once

+ Test new multi-todo functionality.
+ Change name of tests from set_prio to depri

Yes, I'm too tired for this. :/

Guess I'm too tired for this.

Add tests for EditCommand