Add explicit return types to core APIs, other improvements

More minor refactoring in core project (this one will be the last for now).

- add explicit return types to core public methods
- rename org/scalatra/params.scala to org/scalatra/ScalatraParamsImplicits.scala
- add indent to reduce one line length
- rename private Helpers to private ActionResultHelpers
I think this one is just a bug and it will be safe enough to change the behavior.