* add Clock interface and LeakyBucket
* add SimulatedClock
* removed self reference and format
* renamed t to lb
* replaced sync/atomic with uber/atomic
* add source to the license comment
* favor value over pointer receiver for SystemClock
* use atomic.Int value
* use mutex based instead
* add source
* update comments and hide mutex
* add SetRateLimitOnTheFly
* fix: source in rate_test
* skip unstable slack tests on Github
