Adds scalastyle SBT plugin

- See http://www.scalastyle.org/rules-0.5.0.html

We have more contributors to Marathon now, let's make it easy on ourselves and codify the style guidelines.  Much of the existing code could also benefit from a review for style uniformity.

The style checker options are defined in a new XML file called [scalastyle-config.xml](https://github.com/mesosphere/marathon/blob/scalastyle/scalastyle-config.xml).  @drexin @aquamatthias please chime in with opinions.

For example, the proposed config warns for lines over 120 columns long.  While I prefer 80 columns, hopefully we can compromise at some reasonable value.  Ideally we can draw inspiration from the good hygiene in Mesos itself -- where the code is readable and consistent.