Omnirepo

As originally discussed here:

https://groups.google.com/d/msg/celluloid-ruby/SsGjMAtYacU/09tY4ghCd6gJ

This merges Celluloid and Celluloid::IO into a single repository containing both. The history of both is preserved.

Still to consider:

* Celluloid::ZMQ
* DCell
* Timers