Renamed RateLimit struct to TooManyRequests{} and added new event struct
RateLimited{} which can be registerd to with AddHandler() and will be
emitted anytime a HTTP 429 is received on the HTTP API.
AddHandler now returns a func that can remove the handler.
The handlers map is now guarded by its own mutex.
Moved eventMap to events.go for readability.
Improved documentation.