+ (NSDictionary *)JSONKeyPathsByPropertyKey;

I recently used Mantle on a project when converting JSON into model classes, and it was great! I removed a lot of boilerplate code and my 90% of my models had empty implementations. 

However, 

I had to conform to the `MTLJSONSerializing` protocol, which _requires_ that `+ (NSDictionary *)JSONKeyPathsByPropertyKey;` be implemented. This seemed rather ridiculous, as the naming conventions of the JSON objects matched the classes, so I just ended up having to implement this method across all the models and return an empty dictionary. 

This patch follows the philosophy of Mantle and removes unnecessary boilerplate.