Add async.circularBuffer

[pchiusano]

Help wanted on this. This existed in 0.8, I believe @runarorama might have contributed it. I think a reasonable 0.9 signature for it would be:

def circularBuffer[F[_]:Async,A](capacity: Int): F[Queue[A]]

in async package. Behaves like a regular FIFO queue, but if an enqueue would make the queue size greater than capacity, do a dequeue first. Seems like it would be easy to implement in terms of a regular unbounded Queue plus a Semaphore.

[mpilquist]

If no one has started this, I'll add it sometime this week.