pitz: new package

pitz is a distributed bug tracker, inspired by ditz.

The clepy python module is also added, it is a dependency of pitz.

TODO: pitz has a pitz-shell utility that depends on ipython, but when I
enabled it it raised an exception. I think it depends on an old IPython
version:

  from IPython.Shell import IPShellEmbed
  ImportError: No module named Shell

pitz-shell is not the primary interface, it is not critical to have it
working.

There are not many distributed bug trackers out there, so I hope that
adding pitz to nixpkgs may inspire people to support pitz (or similar
software).