Reactive Slick

Here's the ongoing work on Reactive Slick.

/cc @cvogt for review later
Yes, sure, it's used for the asynchronous Action sequencing. And if you call `db.run(a.asTry)`, you will get a `Future[Try_]]` result, but that's not the point of this operation (you could just use Future.onComplete instead). The goal is to get an *Action* of Try[T], so you can do something like `a.asTry.flatMap( /* error handling here */ )` to implement error recovery operators. Action cannot not support all the low-level methods of Future (like onComplete/onSuccess/onFailure) because it needs to enforce sequencing and session handling, so you need higher-level combinators like asTry for this purpose.