make Scheduler use the same clock as IoReactor

[Roguelazer]

This allows one to actually pass a custom clock to IoReactor.new and have it do the right thing. Prior to this, Scheduler always used Time (which would cause problems if using Ione with things that mock time, like Timecop).

This is a big part of fixing #52.

[iconara]

Thank you, this fix is now released as v1.2.5 and v1.3.0.pre3