fix issue where starred repositories were not fetched correctly (0 returned)
implement the 'current_user' mechanism by a singleton class
remove user instance variable from NewsFeedViewController, use CurrentUserManager instead
remove the 'user' instance variable, use CurrentUserManager instead
fix bug where current user is not set correctly
fix bug where user profile information not fetched correctly
add more getters and method to check whether the current user is starring a particular gist
a user can now star/unstar a gist
minor refactor for repo starring functionality