mtl_firstObject now calls forward to Foundation's firstObject.
Merge pull request #110 from steipete/removeFirstObject

Remove mtl_firstObject, declare header so we can already use it in Xcode 4
++specta and expecta
Merge remote-tracking branch 'upstream/master'
[Mantle/remind101] Added reverse transformations via relationshipModelTransformerForKey: so managed object relationships can be properly reverse transformed back to MTLModel objects via NSValueTransformer.