Small speedups in RKPropertyInspector

When doing investigations on #2065, the RKPropertyInspector method classForPropertyNamed:ofClass:isPrimitive: is (unsurprisingly) one of the larger hotspots left, since it gets called extremely often.  It has been optimized pretty well already, so not much can be done that I can see, with one exception.  A simple accessor method call is faster than an NSDictionary -objectForKey:, and the property information is stored with immutable NSDictionaries with known keys, so we can easily convert that to a real object, and eliminate one -objectForKey: call.  That has a small but measurable effect.

I deprecated the previous key constants, though implemented methods such that if the new object is used as a dictionary it should continue to work in most cases.  Not sure if you would prefer that approach or to just remove the existing constants.

I also sped up the rk_classForPropertyAtKeyPath: method similar to a change in another pull request, though that does not get called during my test case.

Timings before:
(Device) Mapping 5000 students with relationship mapping: 16.064542
(Simulator) Mapping 5000 students with relationship mapping: 2.274153

and after:
(Device) Mapping 5000 students with relationship mapping: 15.740304
(Simulator) Mapping 5000 students with relationship mapping: 2.230252
For RestKit operation, no.  But if there is existing code out there making use of the class, then maybe.  That was my question in the pull request comments, which way you prefer to go with that -- have a deprecation route, or just remove the former public API.