Add build scripts and make rules to simplify repo setup and docset generation.

See comments on 9dda21d.

A small .gitignore addition has been rolled into this CL to ignore files generated by the new build scripts.

This is a followup to the following pull request, and is probably best read as a diff off that:
https://github.com/wesalvaro/goog-docset/pull/3
I suppose it would be technically possible for me to factor these changes out separately from that one, but I don't want to deal with testing these changes against the old Closure library repo which is deprecated. It's too bad that GitHub does not have dependent pull request functionality since I'd use that here.