Merge ruby-atomic

## FOR REVIEW -- DO NOT MERGE

This is an attempt to merge the ruby-atomic gem into concurrent-ruby. I used `git subtree` to merge the gem with complete history. I then began moving files and making updates. The C extensions work and all tests pass under MRI. The Rubinius version works and the tests pass under Rubinius . There is something wrong with the Java extensions and the native class isn't loading properly under JRuby. The tests for `Concurrent::Atomic` do not pass under JRuby.

This update moves the `Atomic` class into the `Concurrent` module.

Before we merge we should:

* Decide if this is is the direction we want to go with this merge
* Ask @headius if he is OK with us moving the class into the `Concurrent` module
* Fix the JRuby bug

Thoughts?