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

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

Guess I'm too tired for this.

Add tests for EditCommand

Edit the archive file with '-d' option.

This implements archive option from #2.

Implementation of editing todos in $EDITOR

implements #2

- `topydo edit` will still open todofile in $EDITOR
- `topydo edit 1 2 3` will open tmpfile with corresponding todos
- `topydo edit -e @Home due:today` will open tmpfile with todos matching
   the expression