Forbid ImmediateExecutor for Actors
Merge pull request #175 from ruby-concurrency/actress

Forbid ImmediateExecutor for Actors
All messages should have same priority

It's now possible to send actor << job1 << job2 <<:terminate! and be sure that
both jobs are processed first.
Improving documentation
First draft IVar-Future-Promise-Probe unification
Adding Delay and benchmark of the synchronization