Validate executor field (cull bogus tasks).

Review: https://rb.mesosphe.re/r/441
A Marathon wrapper, to load options from files.

Review: https://rb.mesosphe.re/r/449
Extend proto to store executor.

Review: https://rb.mesosphe.re/r/476
Updating mesos version

Review: https://rb.mesosphe.re/r/494
Merge branch 'master' of github.com:mesosphere/marathon

Conflicts:
	src/main/scala/mesosphere/marathon/api/v1/AppDefinition.scala