added User Class and Friend Class
Merge remote-tracking branch 'origin/master'
refactor UserStore
refactor UserStore and UserManager
modify FriendsController to determine if user is login before showing tableView
refactor UserManager and modified CoreData Store
fix two bugs
1. userObject is not a NSDictionary;
2. GroupName may be NULL, make sure server side return @"" instead of NSNULL