[Biopython] Gene ontology parsing

[Kyle]

> There are already several people working on GO stuff in branches on github,
> e.g. Chris Lasher, Kyle Ellrott, Tamás Nepusz. I don't know if any of them are
> doing OBO v1.2, but it would be sensible to check and try and combine efforts.

The branch at http://github.com/kellrott/biopython/tree/gosupport
should parse most of the information held in OBO v1.2.
Chris's original version was targeted only for the GO OBO file, as
there was a typecheck to make sure the node ID's started with 'GO:'.
That's disable in my branch, and I've used the package to parse a few
of the other ontologies found at www.obofoundry.org.
The module is currently called Bio.GO, but maybe it should be
re-factored to represent the fact that it covers general OBO files,
and not just the GO file specifically.

The main things things keeping it from merging into the main branch
are proper documentation, complete unit tests, and making sure that it
covers all of the standard usage practices.

If you can try it out, and let me know which function are missing (and
maybe contribute some code), we can push this thing forward.

Kyle