(Feat/login page) Finish login page

In reference to issue #17 

I decided not to include the "Create An Account" button since the OAuth buttons should either login or initiate a signup. 

Flow:
* OAuth login with FB or Twitter
* After authentication, app does a DB query to see if user exists and determine if they're a restaurant or a customer. 
  * If user doesn't exist: we redirect to the signup page which should ask if they're a restaurant or customer. Depending on which one they click, we serve them the appropriate signup form to enter any additional information Twitter/Facebook doesn't pass back to us. 
  * If user does exist: we should query DB to see if they're a restaurant or customer and then redirect them to appropriate page (Customers would go to `search-criteria.html`, and Restaurants would go to `availability.html`). 

@lmle what do you think? If you agree, we should run it by Eddie and Armando to get it on their radar. I'll update the balsamiq doc in case it's easier to see visually.