Code review comment for lp:~jml/gtg/doc-generation

This branch adds 'make apidocs' and 'make edit-apidocs' targets to the Makefile and updates the HACKING file to explain how docstrings & API docs should work. I've also updated the README file to indicate the new optional dependencies.

To build the docs, you'll need pydoctor & its dependencies. pydoctor is best retrieved from lp:pydoctor. If you can't figure out how to get it running from the README & HACKING docs, then the documentation needs to be improved.

The rest of the branch is fixes to docstrings that we found when I tried to generate the docs.

An example build of the API docs can be found at http://static.mumak.net/gtg-api/.