Func0 can transparently implement java.util.concurrent.Callable.

This change doesn't change the API at all for users of Func0, but it makes all Func0 objects immediately reusable with any JDK API that accepts Callables. For example, a Func0 can now be submitted directly to an ExecutorService for asynchronous execution. It also allows the elimination of a small amount of redundant code within RxJava itself.
Merge pull request #1003 from rickbw/callable-func0

Func0 can transparently implement java.util.concurrent.Callable.