Added method to RKManagedObjectStore.

Added a method to use existing app MOC instead of creating new ones.

- (void)usePersistentStoreManagedObjectContext:(NSManagedObjectContext *)moc andMainQueueManagedObjectContext:(NSManagedObjectContext *)mainMoc;
Added a method to cancel all operations, including in-execution.
Merge branch 'development' of github.com:taptaptap/RestKit into development