Better documentation of RubyExecutor and JavaExecutor.
ImmediateExecutor is now a full executor service.
Created SerializedExecutionDelegator to wrap SerializedExecution.
Created SerialExecutor 'type' module for interrogating objects.
Fixed Yardoc for Atomic
PerThreadExecutor is now an executor service.
Removed sleep statements from Event specs.
Removed sleep statements from Event specs.
Improved specs and docs for ImmediateExecutor and PerThreadExecutor.
Better executor documentation.
Refactored executor specs.
Merge pull request #121 from ruby-concurrency/refactor/executors

Refactor executors