Removed JavaAbstractThreadPool in lieu of JavaThreadPoolExecutor.

* Created JavaThreadPoolExecutor; some specs failing.
* JavaThreadPoolExecutor specs passing.
* Added shared specs for global thread pool.
* Removed JavaAbstractThreadPool in lieu of JavaThreadPoolExecutor.
Merge branch 'master' into thread-pool-executor
Created stubs for RubyThreadPoolExecutor.
Implemented RubyThreadPoolExecutor.
Ruby thread pools now extend RubyThreadPoolExecutor.
Removed duplicate code in thread pools.
Refactored thread pools.
Merge pull request #39 from jdantonio/thread-pool-executor

 `RubyThreadPoolExecutor` class