Integrate obfs4proxy into the TBB bundle.

I did some of the preliminary work based off dcf's meek branch a while ago, now that meek has been merged into TBB, and obfs4proxy is approaching what I consider deployable, it is the time to change my patch into a real branch.

Open questions that I need to figure out:

• Is it ok to just mirror dependencies at people.tp.o/~yawning/mirrors/source?
• My patch only edited versions.nightly since I wasn't (and still am not quite) at the point where I was comfortable tagging obfs4proxy. Is there a way to say "use this commit" for the other files?
• The LICENSE file needs to be copied.