Create an IndirectImmediateExecutor
Implement IndirectImmediateExecutor with just one PerThreadExecutor
Add specs for IndirectImmediateExecutor characteristics
Merge pull request #167 from pitr-ch/pools

Allow to disable auto termination of global executors
Merge pull request #166 from obrok/indirect-immediate-executor

Create an IndirectImmediateExecutor
Add Promise.zip
Merge pull request #168 from obrok/promise-zip

Add Promise.zip
Better docs on Future.
Merge pull request #174 from pitr-ch/pools

Fix leaking threads when pruning the pool
Forbid ImmediateExecutor for Actors
Merge pull request #175 from ruby-concurrency/actress

Forbid ImmediateExecutor for Actors
Add missing synchronizations to TimerSet