Add Mnemosyne 2.2.1.

The Mnemosyne Project has two aspects:

  * It's a free flash-card tool which optimizes your learning process.
  * It's a research project into the nature of long-term memory.

We strive to provide a clear, uncluttered piece of software, easy to use
and to understand for newbies, but still infinitely customisable through
plugins and scripts for power users.

Mnemosyne uses a sophisticated algorithm to schedule the best time for
a card to come up for review. Difficult cards that you tend to forget
quickly will be scheduled more often, while Mnemosyne won't waste your
time on things you remember well.

If you want, anonymous statistics on your learning process can be
uploaded to a central server for analysis. This data will be valuable to
study the behaviour of our memory over a very long time period. The
results will be used to improve the scheduling algorithms behind the
software even further.
Include Mnemosyne in nix packages collection.
Patch *all* references to `sys.exec_prefix`.
Have Mnemosyne take pkgs, rather than pythonPackages as argument.
Mnemosyne: provide capacity to override dependencies at the fidelity of a Python package.
Merge branch 'master' of git://github.com/fmap/nixpkgs

Add Mnemosyne 2.2.1