Update to new Github closure-library repository

As announced on closure-library-discuss [0], the canonical Closure library repo is now hosted on GitHub, and in fact the old code.google.com version will eventually go away [1]. In order to take advantage of GitHub Pages, Closure Library docs are now hosted on the gh-pages branch at the new repo:

https://github.com/google/closure-library/tree/gh-pages

This CL updates the git submodule definition to point to the new repo, and tweaks the gendocset.py script to work with the new repo layout.

[0] https://groups.google.com/forum/#!topic/closure-library-discuss/PHoeRUvExHc
[1] https://groups.google.com/forum/#!topic/closure-library-discuss/4Z7VquHzNuk