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
Merge branch 'master' of https://github.com/gerasim13/RestKit