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

* 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.
* 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
Merge remote-tracking branch 'origin/master' into topic/closerToSlick

Conflicts:
	src/main/scala/play/api/db/slick/SlickExecutionContext.scala
reintroduces Config object for clarity, make Database compare well (by making it an ordinary class)
renames files for clarity, improves slick hook comment
Merge pull request #60 from cvogt/topic/closerToSlick

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