Concurrency classpath extension (part of the atomic package implementation)

Here is an implementation for AtomicBoolean, AtomicInteger, AtomicLong, and AtomicReference.  Thanks to Joel for doing the hard work, these were all pretty straight forward to implement.

The unit test basic confirm that compare and swap is being used correctly, and works correctly.  I verified it with much higher numbers, but reduced them to keep unit tests fast.