fixes #476

- made statuses field on MarathonTask obsolete
- added status field that contains only the last status
- added migration for status field changes
- added `status` and `status[]` params to /v2/tasks
- updated docs

Conflicts:
	docs/docs/rest-api.md
	src/main/java/mesosphere/marathon/Protos.java
	src/main/scala/mesosphere/marathon/MarathonSchedulerActor.scala
	src/main/scala/mesosphere/marathon/api/EndpointsHelper.scala
	src/main/scala/mesosphere/marathon/api/v2/TasksResource.scala
	src/main/scala/mesosphere/marathon/state/AppDefinition.scala
	src/main/scala/mesosphere/marathon/state/Migration.scala
	src/test/scala/mesosphere/marathon/tasks/TaskTrackerTest.scala