Adds FB Login for users

Adds FB Login + Specs
@witbok put these into ENV. We are using the dotenv gem, so you make a file called `.env` in your project root and add something like

```
FB_SECRET=111222333
FB_SECRET_KEY=123123123123
```

I'm not sure what those numbers are exactly, so you may end up renaming them appropriately. You should also invalid your facebook keys because they're on github now.