Expanding avian's classpath for java.util.concurrent

I have been thinking about expanding avian's classpath for java.util.concurrent.  This is just a small first step in that direction.

I wanted to send a pull request and see what others thought about this before I go too much further, but if others are interested I can continue this work.

Right now this is just adding interfaces and low hanging fruit for what would need to be expanded in the future.  If we do want to merge this, I can also add unit tests for the implementation of TimeUnit.

Let me know your guys thoughts on if I should continue to implement this for avian.  One possible thought would be that if I had enough java.util.concurrent implementation in Avian, I could get my Threadly project to compile for the avian classpath, and allow for quite a range of concurrent utilities.  That is my primary goal here, expand avian enough so that Threadly can possibly be used within it.