Reactive Slick

Here's the ongoing work on Reactive Slick.

/cc @cvogt for review later
@julienrf That looks like a different problem. The issue here is not higher kinds but the fact that you have an `AsTryAction[T](a: Action[T]) extends Action[Try[T]]` (simplified) and pattern match on an `Action[R]`. There's no way to pattern-match on this and enforce `R == Try[T]`