Add explicit return types to core APIs, other improvements

- 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
BREAKING: Change Richrequest#locales to return Seq[Locale] instead
BREAKING: Change org.scalatra.util.using returns A value (previously returns Unit)
JUst to be safe, care nullable #getLocales inside RichRequest#locales
Merge pull request #456 from seratch/explicit-return-types-in-core-project

Add explicit return types to core APIs, other improvements