Implemented AtomicXXX and YYYReferences in javalib, including tests

This is all I need to get [scala.rx](https://github.com/lihaoyi/scala.rx) working flawlessly in the browser! It's tests all pass for `publishLocal`ed version of scala-js.

The `WeakReference` family is fake, but I figured having non-working weak references (which you can still clear manually) beats out forcing people to re-implement their custom reference class and do the annoying switching between them on cross compilation. Even if you did that, the API would be 100% identical as the broken-weak-references anyway