Go rate limiter

Package provides an implementation of the leaky token algorithm

• Accepts func through a Commands channel and GOrunning it if bandwidth allows
• If refused calls Cancel() method, or sends Command back if a return channel exists
• Limits:
  1. n of max concurrent commands – BurstRate
  2. n of commands per rate-limit time – CMDLimitPerTime
• Custom rate-limit time (per second/minute/..); default: per minute
• Time "after to check" of new tokens issue; default: 1 ms

NB: don't forget to close a Commands channel