Added support for custom working directories

A solution to issue #790
It seems that the implementation of the following two methods are functionally identical:

 - `- (void) MR_addiCloudContainerID:(NSString *)containerID contentNameKey:(NSString *)contentNameKey localStoreNamed:(NSString *)localStoreName cloudStorePathComponent:(NSString *)subPathComponent completion:(void(^)(void))completionBlock;`
- `- (void) MR_addiCloudContainerID:(NSString *)containerID contentNameKey:(NSString *)contentNameKey localStoreAtURL:(NSURL *)storeURL cloudStorePathComponent:(NSString *)subPathComponent completion:(void(^)(void))completionBlock;`

Could you combine these somehow?