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.

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

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

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

Also specifically indicate Python 2.7, since Python 2.6 is not
supported.

The icalendar package does not support Python 3.2 throwing a
SyntaxError.

Also fix the ical test case that reads the test data incorrectly.
Windows newlines were not properly decoded, resulting in a mismatch
between the output of the icalendar package and the test data file.

(Implicit in Python 2, has to be made explicit with // in Python 3).

Quite a puzzle to get this right for Python 2 and 3 simultaneously...

Text identifiers were calculated with the hash() builtin. The problem is
that hash() returns different values depending on Python's major
version or 32 vs 64 bit. With Python >=3.3, the seed for the hash
function is always different (for security).

Instead of relying on hash(), use the sha1() hash to obtain a text
identifier.

Still some test cases are failing, but the basic functionality seems to
werk.

$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.

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