Move authentication to Oauth2

**NOT READY FOR MERGE.**

There's still a bit more cleanup to do. Will update this with a checklist by the end of the night.

Feel free to bring up whatever you see, but be aware that I know a few things are hacky/misnamed right now. Just wanted to get this out there so you can get a feel for what the major changes will be.
https://code.google.com/p/webapp-improved/issues/detail?id=42