Fix broken Barnes-Hut simulation.

A data race where bodies where concurrently read and modified in the
`updatePosition` method should now be eliminated.

Also tweaked `.gitignore` and fixed the build file so that we can
use the `bench` task.
Merge remote-tracking branch 'github/master' into topic/barnes-hut-fix

Conflicts:
	project/Build.scala
Merge pull request #7 from DarkDimius/topic/barnes-hut-fix

Topic/barnes hut fix