Minor Fixes + Improvements

- Added `RKJSONDictionaryGetObjectAtKeyPath` function to facilitate cleaner accessing of nested JSON-parsed dictionaries.
- Added `RKAwait` function suite to allow synchronous realization of promises.
- Added default refiners and matchers to `RKPossibility`
- Cleaned up and optimized `RKBinding`.
Small change to exception-throw RKAwait.
Bug fixes.

- `RKURLRequestPromiseCacheManager` is now asynchronously initialized.
- `RKReachability` now posts notifications.
- Added decorator methods returning RKPromises.
Small change to preflight API