start of a Pants-with-scala page

+ points out sample code

+ java_sources, circular java-scala deps

+ mentions 'goal repl'

Testing Done:
rendered at http://pantsbuild.github.io/lahosken-demo/scala.html

Bugs closed: 26

Reviewed at https://rbcommons.com/s/twitter/r/1092/
Sync of https://github.com/pantsbuild/pants/commit/6fe0a53e5b273ea451d65fd3c3ffe1bb67fdad02