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.
Merge branch 'release/0.21.0' into development
Do not attempt to continue property mapping skipping logic in the event there is a `nil` value on the property being mapped.
Merge pull request #1526 from jonnolen/limit_operation_responseDescriptors_to_only_relevant_descriptors

Create RKObjectRequestOperations with relevant ResponseDescriptors only
Cleanup unused function warnings when building without Core Data
Bump RKValueTransformers submodule reference
Update projects to Xcode 5 recommended settings
Fix deprecation warnings and use RKValueTransformer API's for configuring date parsing
Fix bad links in the documentation breakout
Bump dependencies to fix build
Fix issue with objectManager references in test helper
Add conditional compilation around `connectionTestExpectation` to avoid unused variable warning
Fix inability to complete value transformations when using a nested mapping. closes #1656
Bump version to 0.22.0
Update RKValueTransformers to v1.0.1
Update TransitionKit dependency to 2.0
Update CocoaPods to 0.28.0
Bump Pod dependencies
Add support for serializing values to null. closes #669, closes #1311
Bump TransitionKit ref