python: add python.interpreter attribute
buildPythonPackage: add python to propagatedBuildInputs

Doing:

$ nix-shell -p pythonPackage.pyramid

now also sets $PYTHONPATH. Previously you'd have to do:

$ nix-shell -p pythonPackage.pyramid python
Initial documentation for Python packaging. Feedback welcome.

cc @chaoflow @offlinehacker @garbas @goodwillcoding @brodul
@cillianderoiste @matejc

Fixes #64
Merge branch 'python/docs'