added FacebookSDK; added login and user friends permission process with facebook
removed UserInfo and stores directly into PFUser; linking a facebook account doesn't create a new anonymous user
uses Parse's email validation on signup; moved facebook calls to FacebookHelper
if facebook permission is denied or user cannot be linked, logs out of facebook session
textfields on signup become first responder automatically
Merge pull request #3 from mitrenegade/connectFacebook

Connect facebook