Add support to fire callback immediately when rate limiting

This module was perfect for my needs, except I need to alert the client that rate limiting is in effect instead of just delaying the callback, so I added support for this in the RateLimiter. It simply fires the callback with remainingRequests set to -1, so that the fact that rate limiting is in effect can be easily detected from within the callback.

I also took the liberty of adding javascript code wrappers when updating the README.