Fix sync of related objects

This commit fixes how related objects are fetched from Parse. To-one
objects were successfuly synced before, however this commit adds a cal
to PFRelation to fetch the to-many objects. Additionally, to prevent
infinite recursion, fetched objects will cease to fetch related objects
if they are optional or have already been synced.
Merge pull request #24 from sbonami/23-Fix-sync-of-faulted-related-objects

Fix sync of related objects