Begin Version 3, Stable

Version 3 can be considered stable. No interfaces in this version will be altered or removed in subsequent versons.

- RKGenerateSongID has been obsoleted. Use RKGenerateIdentifierForStrings instead.
- Removed RKMultiPartPromise. The interface was ill-conceived as it did not lend itself to stable composition. Multi-part promises are still acknowledged in RKPromise through the method `-isMultiPart`. No RK code uses or supports multi-part promises.
- RKURLRequestPromise is no longer a multi-part promise. Added a method to replace the first success from the former multi-part contract.
- Removed propagation support from RKBinding. The system provides this.
- Added RKObjectRetain, RKObjectRelease, and RKObjectAutorelease methods to assist in circumventing the compiler's prohibition on manually managing retain counts.

- Fixed a crasher in RKPossibility.
- The RKURLRequestPromiseCacheManager class now returns nil and no error for non-existing cache.