Generate docs for Python overrides

[Mikkel Kamstrup Erlandsen]

An important part of many pygi based APIs are the overrides which typically wraps varargs methods and adds Pythonic list comprehension and such. Currently the API provided this way is not included in the generated Python docs.

I'm not really sure how to solve it as this info is not available from the GIR - but maybe we can load in the override module (since Giraffe itself is written in Python) and work on Python's ast of the override and merge it in to giraffe's ast. There's room for a fun hack here at least :-)