Fix #19 by returning NULL on non-loaded relation

 * Added 'entity()' method to single relations and Entity object so you
   can always call '$relation->entity()' without error, even if a loaded
   Entity object had already been set over the temporary relation object
 * Added more class checks to ensure specified relation class is a valid
   Spot\Entity object
Merge pull request #22 from vlucas/ticket_19

Fix #19 by returning NULL on non-loaded relation