Refactor Pools using message API

While working on my own background queuing solution, WindUp, I ended up
reimplementing Celluloid.pool using the message API. The result is a drop-in
replacement for pools, with a few changes:

* Asynchronous message passing - using Celluloid's Messaging API means
  any blocking call (i.e. #sleep, Celluloid::IO, etc) allows the actor to
  process another message
* Separate proxies for PoolManager and pools - no more unexpected behavior
  between #is_a? and #class and #responds_to?

Separating the proxies *may* cause some incompatabilities; I haven't yet found
any, but the separation of concerns may have unforseen side effects here.