replaces play-slick's custom DB interface by Slicks standard one

This PR is based on https://github.com/freekh/play-slick/pull/59 (which is why the commit shows up here).

* refactored play-slick DB/Config classes into Database. This means minor simplifications with API and imports.
* removed play-slick custom DB interface and inherited from scala.slick.session.Database instead. This makes slick-play apps compatible with ordinary Slick, which would for example allow shared code between a slick-play app and a pure slick app. This also means that DB.withSession{ implicit s:Session => ... } requires a type annotation of Session, just as Slick currently does. We will change this in Slick in the future, so it will not be necessary anymore.
* moved DBAction out of the cake using AsyncResult instead of Async. This makes it more universal and it just comes with the import, not need to inherit a slice. Be aware, that AsyncResult and Async will both be deprecated in the next play version in favor of Action.async (at least that's what current master says).
* minor refactorings of slick-play