Correct README about firebase API license.
Update firebase to last version that was (accidentally!) released as MIT.
[https://groups.google.com/forum/#!topic/firebase-talk/pAklVV3Whw8]
Explain in README why I'm now pinning firepad and firebase,
and drop their branch-tracking from .gitmodules (ineffective?).
Fix #58 (errors on incoming changes from other instances).