Scalatest + Play docs 2.3.x

This pull request adds documentation for ScalaTest + Play library, which is now released 1.1.0-RC1, to the Play documentation for the Play 2.3.x branch. The Play docs now give a tutorial and examples for both ScalaTest and specs2. The names of the relevant pages have been renamed, but all start with "Scala." All links have been updated. (It passes validate-docs.) Source code is imported and tested. To test just the ScalaTest code, because ScalaTest + Play is an external module, you need to use this command:

./build -Dexternal.modules=scalatestplus-play test