Small speedups in RKPropertyInspector.  Regular objects are faster than dictionaries, so for the actual property information using a simple method call instead of -objectForKey: has a small but measurable speedup.  Deprecated the previous key constants, though implemented methods such that if used as a dictionary it should continue to work in most cases.
Updates per PR comments.  Remove deprecation path; this is a slightly incompatible change now, though RestKit usage is fine.  Change to use dot syntax for properties, and use inspector local variable.  Also speed up the NSManagedObject override of rk_classForPropertyAtKeyPath: which I had missed before.
Merge pull request #2083 from carllindberg/2065-RKPropertyInspector

Small speedups in RKPropertyInspector