Add the capability to autodetect the current user

The autodectection of the current user is done through
two ways.

1. If a request calls /me/*, it tries to get the identifier
   from that request.
2. Manually, but calling the private method
   updateCurrentUserIdentifier. This method will then perform
   an arbitrary request through arbitraryRequest and get the
   identifier associated to the current user.

A new property, currentUserIdentifier, has been introduced
to FacebookInterface. If no identifier has been retrieved yet,
it's value is "me".