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/

![screenshot from 2013-06-23 14 20 46](https://f.cloud.github.com/assets/393494/692789/cce8305a-dc4a-11e2-9b69-87464ab77ef8.png)

I'll submit another pr with the Twitter functionality hacked on soon.

Anyways, love the project and tech stack. Had to contribute.

Best,

Josh