Add rate limiting

This pull request adds support for rejecting incoming connections if too many goroutines are active at once, and rate limits HTTP requests from origin IPs. `RateLimitedHandler` uses a token bucket implementation ported from [`node-restify`](https://github.com/mcavage/node-restify).
These should probably use a sync.atomic operation on an int pointer. The int can then keep track of exactly how many clients are connected atomically and correctly.