SPARK-4375. no longer require -Pscala-2.10 and place external modules behind profiles

It seems like the winds might have moved away from this approach, but wanted to post the PR anyway because I got it working and to show what it would look like.
Yes, and yes definitely.  This is the one thing I'm still unsure about.  But I still haven't been able to successfully build with Scala 2.11.  Even without any of my changes.