Add `AtomicReference` as in Atomic gem

[pitr-ch]

Implement simple AtomicReference with locks or possibly merge Atomic gem, came up in #73.

[chrisseaton]

I've done a simple mutex version of this in f187c6f so you can start using it. We'll look at using Java primitives (including the new Unsafe.getAndSetObject from Java 8) soon.

I've called this just Atomic for now, as of course it works with all objects. AtomicReference is too long, and AtomicRef isn't very Ruby-like. What do other people think?

[jdantonio]

@chrisseaton Thank you!

[pitr-ch]

@chrisseaton Thanks! I know I am a rookie here, but please open these as PRs next time. It's much easier to discuss the changes then. I'll add comments to the commit.

[jdantonio]

Now that we have an initial implementation and are working on a plan to merge the ruby-atomic gem, I'm closing this issue.