Optional test dependencies separation

This feature allows a user to separate out test-scoped dependencies from their Play2 build by specifying an optional parameter `testLib`.  This of course must be accompanied with a corresponding sbt config, e.g.:

```scala
   unmanagedBase in Test := baseDirectory.value / "test-lib"
```
See the SBT docs for more info on such configuration: http://www.scala-sbt.org/release/tutorial/Library-Dependencies.html.

The isolation of test-scoped dependencies is important for ensuring the final Play dist does not contain any test libraries.  Hope you find this useful.  I'd love to see it added so I can use it in professional projects.