Async AutoSubscriber

Hi Mike,
I tried jonnii's suggested expression tree trick here https://gist.github.com/jonnii/4714812 but we don't know the type TMessage of Func < TMessage , Task > at compile time, so I couldn't get that to work.

Also the unit test is very basic as I haven't figured out how to inspect the type arguments on the onMessage parameter.

This definitely needs some improvement before merging in.

-Dan