Added Thread Context bits

Sometimes you get exceptions in Callables and Runnables and it would be really nice
 if you could see the trace path that got them on the executor.  The Context class should
help with this.
Actually, turns out that you might want to do `new Throwable().getStackTrace()` instead. It's supposedly more than 10x faster than `Thread.getStackTrace()` and also includes the current method stack frame.

http://bugs.java.com/bugdatabase/view_bug.do?bug_id=6375302