Reactive Slick

Here's the ongoing work on Reactive Slick.

/cc @cvogt for review later
Nope, it's not `fallbackTo`, either. `AndFinallyAction` always returns the result of the first Action and always runs the second Action, preferring the failure of the first one if both fail. It's the asynchronous version of
```
try a1 catch {
  case NonFatal(ex) =>
    try a2 catch { case NonFatal(_) => }
    throw ex
}
a2
```