Scheduler implementation

[ioquatix]

This is a prototype of the scheduler which uses the interface provided by ruby/ruby#3032

At the present time, the scheduler is an optional set of hooks for implementing non-blocking IO. However, in the future, the scheduler might form the core of the reactor, replacing the selector entirely.

[coveralls]

Coverage decreased (-11.0%) to 85.185% when pulling b5a38d5 on scheduler into ae19d9a on master.