add methods for pending friend requests.
set up paths in FriendsResource to get list of pending friend requests
add mongo methods to identify non-bidirectional friendships
add Resource method to decline a friend request.
remove friends bi-directional as invoked by an explicit request
Merge pull request #164 from matthesrieke/feature-pending-friend-requests

Feature pending friend requests
expose Link header with Access-Control-Expose-Headers
Merge pull request #166 from autermann/cors-link

Expose Link header with Access-Control-Expose-Headers
Merge pull request #167 from enviroCar/dev

merge recent developments