Value Transformer Architecture

This pull request adds a new subsystem to the object mapping engine that enables the use of pluggable value transformers. All value transformations that were previously handled within the internals of `RKMappingOperation` have been factored out into simple standalone transformers. The transformer API is completely extensible and customizable and transfers full control of the mapping process from the framework back to the end developer.

The API has the following design properties:

* Transformers are implemented via a simple `RKValueTransforming` protocol. The protocol has a single required method: `transformValue:toValue:ofClass:error:` and an optional method for validating transformations `validateTransformationFromClass:toClass:`.
* To ease implementation, a concrete `RKValueTransformer` class is provided that enables the end-developer to implement a transformer via blocks.
* All previous internally implemented transformations are provided as singleton instances on `RKValueTransformer`.
* To facilitate transforming a value into an unknown target representation with an arbitrary collection of value transformers the `RKCompoundValueTransformer` class is provided. This class itself implements `RKValueTransforming` and will attempt to transform an input value with each of the transformers it contains in programmer defined order, enabling a simple transformation chain to be expressed.
* The previous Date and Time formatting API's have been harmonized with the new value transformation API by making `NSDateFormatter` conform to the `RKValueTransforming` protocol.

It is notable that this API intentionally does not make use of the `NSValueTransformer` class provided by Cocoa. The API was not a clean architectural fit with the needs of RestKit and the implementation contained in the PR is quite a bit simpler and more straight-forward than an `NSValueTransformer` based implementation would be (we walked quite a ways down this road before changing gears).

Full test coverage is implemented and API documentation should be adequate. Comments are feedback are most welcome.