v0.2.0

Additions:

1. Now we can call defer limiter.Kill() to clean up or deactivate the limiter when it is not in use anymore. This will also kill the underlying goroutine and avoids resource leak.
2. limiter.ShouldAllow(N uint64) now returns bool, error. error is not nil if ShouldAllow is called on a limiter that is killed or inactive.