[#673] - log4j still works, juli still works but can now switch to logback.xml

This pull request is a good compromise so people who want to use logback can while not breaking log4j, slf4j, juli logging.

NOTE: please read the comments in the ticket I added at the bottom of http://play.lighthouseapp.com/projects/57987/tickets/673-feature-request-use-logbackslf4j-instead-of-log4j

since it documents an issue that current playframework has and this new logback implementation fixes.