Fix bug with dynamic parameterization when the concrete mapping includes a date.

Instead of relying on reference to the RKMapping, which could be dynamic, traverse from the attribute mapping back to its parent to obtain the appropraite date formatter.
Don't throw an exception if doing a simple attribute mapping with a nil data source. closes #1005
Improve assertion error message
Add global __RESTKIT__ define for aiding conditional compilation
Refactored relationship mapping implementation to make it easier to follow and debug
Added assertion to trap very nasty Core Data mapping error in which `existingObjectWithID:error:` returns an object in another context, indicating that the object graph has been poisoned, most likely by obtaining a permanent managed object ID from the wrong context.
Implement support for cancellation and prioritization of response mapping operations using an operation queue global to all object request operations.

* Passes through cancellation from object request operation to HTTP request operation and response mapping operation. closes #993
* Performs all object mapping within an NSOperationQueue to enable constraining of mapping activities.
* Add default mapping queue with concurrency limit of 1 operation.
* Migrate deserialization of the response body into a serial dispatch queue to ensure that only one parse occurs at a time.
Improve docs on success/failure blocks
Update stubbing routines for fetch requests to support copying of fetch request blocks that require parameters to construct a matching URL
Add support for connection mappings to the RKManagedObjectImporter.
Fix bug in which relationship connection operations are not enqueued for `RKMappingOperation` instances whose mapping is an `RKDynamicMapping`
Fix a bug in previous commit
Add assertion that gives the developer a hint of how to fix an attempt to add an array of attribute mappings that already exist in another mapping
Add error condition when attempting to map an RKEntityMapping containing connection mappings with a nil managed object cache.

* Add support for the data source to fail mapping operation with an error.
* Clean up error handling within RKMappingOperation
Fix horrible bugs in previous commit
Drop extraneous space between class and object address in property description
Added assertion and cancellation hardening within the Search Indexer
Change invalid `addAttributeMapping: reference to correct `addPropertyMapping:` in README.md
Add a decent description method to the object request operations
Cancel response mapping operations during tear down