Merge branch 'release/v0.20.0-rc1'
Merge branch 'release/0.20.0'
Drop pre from README.md install instructions
Change TransitionKit submodule url to generic url.

The url had blakewatters as user and required his keys.
Merge pull request #1357 from yanik/development

Change TransitionKit submodule url to public url.
Add performance optimization when there is only a single identification attribute in use. Switch to `compare:` instead of `localizedCaseInsensitiveCompare:` to speed up generation of cache keys.
Eliminate use of `NSCache` within `RKPropertyInspector`. Profiling in instruments reveals that use of `NSCache` is significantly slower than `NSMutableDictionary` for this performance critical code. refs #1232
Add an autorelease pool around the hot mapping loop in `RKMapperOperation` as significant time was being spent in autorelease during mapping and it appears to have a beneficial effect on performance. refs #1232
Back `RKEntityByAttributeCache` with an `NSMutableSet` instead of an `NSMutableDictionary` for performance. refs #1232
Add a heuristic to determine if we can skip merging a `NSManagedObjectContextDidSaveNotification` to avoid merging changes that are already available. refs #1351
Add support for filtering inversion via a block callback
Introduce a new method for conveniently returning all enqueued `RKObjectRequestOperation` instances that match a given HTTP method and path pattern. closes #1356

This change generalizes the functionality used internally by `cancelAllObjectRequestOperationsWithMethod:matchingPathPattern:` and provides a convenient means for determining if any operations are already enqueued and retrieving them for manipulation.

Signed-off-by: Blake Watters <blakewatters@gmail.com>
Guard `RKPropertyInspector` cache with a GCD queue to preserve thread safety
Rework the managed object importer so that you can benchmark different cache backends. refs #1232
Introduce additional experimental performance optimization. refs #1232

* Add `RKMappingOperatationDelegate` method to enable the data source to instruct the operation to skip performing value comparisons when assigning properties
* Add `RKMappingOperatationDelegate` method to enable the data source to instruct the operation to skip property mapping all together
* Add `modificationKey` attribute to enable the property assignment to be skipped when mapping entities that have not changed
Sending a nil propertyClass to classForPropertyName results in putting a nil object in a inspectionCache causing an app to terminate.
Replace `NSRecursiveLock` with dispatch queue in `RKEntityByAttributeCache`. refs #1232
Merge pull request #1362 from cfis/fix_property_inspection

Fix Crash in RKPropertyInspector
Fix incorrect model naming at top of README.md

Rename `Tweet` to `RKTweet`. closes #1370
Sending a nil propertyClass to classForPropertyName results in putting a nil object in a inspectionCache causing an app to terminate.