Provide notification mechanism to enable logging of API usage stats about mbxmapkit

One way to do this would be adding some delegate callbacks to the API, but using NSNotificationCenter (Apple docs, NSHipster) also seems like an attractive option.

I've investigated this extensively, as I described at #73 (comment), and it does not seem to be possible with NSURLCache to tell the difference between a response from a local cache hit and a fresh response that just came over HTTP. It's looking like the only options for accurately counting API hits would be

1. Don't do any performance caching
2. Implement our own caching instead of using NSURLCache

Both of those appear to have a pretty huge downside which seems to outweigh any potential benefit for implementing an API hit counter.