Explicitly defined emptyImage as a strong @property

This is the default under ARC, but the compiler might complain about the lack of attribute when the header is included into files not compiled under ARC
Removed deprecated FBGraphBasePath. Now uses FBRequest and FBRequestConnection to build the query URL.
Reverted #import AFNetworking statement to its original value
Merge pull request #6 from FlorianDenis/master

Fixed some warnings (undefined attributes for properties, deprecated FacebookSDK method)