Facebook and Twitter login

I've added full functionality for an optional Facebook login/register via Parse SDK. The Twitter button is there, but does not work. Built-in functionality for Parse/Twitter sign in only seems to exist for iOS and Android SDKs: http://blog.parse.com/2012/02/27/parse-now-supports-sign-in-with-twitter/

I'll submit another pl with the Twitter functionality hacked on soon.

Anyways, love the project and tech stack. Had to contribute.

Best,

Josh