SBT Cleanup

Major changes:

* Removed code duplication in Plays build file
* Separated concerns of Plays build into different files
* Implemented Play create-dist to replace much of what was done in package
* Implemented repository builder that only produces valid repositories, not just copies the cache
* Made the play run command a task, so that sub projects will now work much better

@gissues:{"order":75,"status":"inprogress"}