adding new ObjectManager methods for building RKObjectRequestOperation objects that take an array of response descriptors.  Updated appropriateObjectRequestOperationWithObject:method:path:parameters: to pass only the response descriptors that match the object, path, and method passed in.
Fixed nil error reference
Merge pull request #1569 from khaullen/error_reference_fix

Fixed nil error reference
changed Error code when response mapping declined with `willMapDeserializedResponseBlock` block
Small stylistic cleanup
Remove SOCKit dependency from Testing module

* Eliminate need to compile SOCKit by using KVC instead of reopening the class
* Remove SOCKit dependency from Testing subspec
Bump to latest revision of RakeUp and CocoaPods
Introduce the `RKValueTransformer` API providing extensible value transformation to the Object Mapping layer of RestKit. closes #294

This changeset introduces a new API that enables the complete customization of the RestKit object mapping process. All previously
internal value transformations have been transitioned into composable value transformer objects. Value transformers are defined via
a simple Objective-C protocol called `RKValueTransforming` and the library includes a concrete block-based implementation of the protocol
via the `RKBlockValueTransformer` class. Transformers are composable and can be assembled into a transformation chain with precedence based
on index order via the `RKCompoundValueTransformer` class.

Transformers are integrated with the `RKObjectMapping` and `RKPropertyMapping` classes via a single `valueTransformer` property that specifies
what transformer the object mapper should use while mapping a given value.

The core functionality of `RKValueTransformer` is fully reusable and will be made available as a standalone library in the near future.
Merge pull request #1588 from RestKit/feature/RKValueTransformers

Value Transformation Architecture
RKObjectMapping copyWithZone should use initWithClass
Merge pull request #1594 from mjarvis/development

RKObjectMapping copyWithZone should use initWithClass
Migrate RKValueTransformers and ISO8601DateFormatter into standalone libraries
Remove warning log message that is likely to produce log flooding
Fix issue with inappropriate handling of managed objects using `nil` key path. fixes #1596
Switch `RKValueTransformers` and `ISO8601DateFormatterValueTransformer` to HTTP submodule URL's
Fix memory leak in state machine. closes #1593
Fix documentation issue on `addAttributeMappingToKeyOfRepresentationFromAttribute:` and transparently transition to using braces instead of parentheses for denoting variables in key paths. closes #1449
Merge pull request #1570 from almassapargali/development

changed Error code when response mapping declined with `willMapDeserializedResponseBlock` block
Rename and deprecate some poorly named properties on `RKObjectMapping`
Bump VERSION to 0.21.0