Reactive Slick

Here's the ongoing work on Reactive Slick.

/cc @cvogt for review later
No, Action[Try] leads naturally to Future[Try]. But I realized last night (after spending a few hours unsuccessfully trying to find a nice way to make `transactionally` work with streaming) that `asTry` is not useful for this sort of thing, and neither is `andFinally`. I need a combinator similar to Future.transform but without a way to see or modify a success value (i.e. it takes an `Option[Throwable] => Option[Throwable]`). `asTry` may still be useful for more complex error recovery combinators (which will require nested atomic Actions) where you don't necessarily need streaming.