Add Support For Max Consecutive Failures on a Task

We now can set the max number of consecutive failures that a task can
have before we kill it. Such number is optional and applies to a each
specific Health Check.

Conflicts:
	src/main/java/mesosphere/marathon/Protos.java
	src/main/proto/marathon.proto
	src/main/scala/mesosphere/marathon/MarathonScheduler.scala
	src/main/scala/mesosphere/marathon/MarathonSchedulerService.scala
	src/main/scala/mesosphere/marathon/health/HealthCheck.scala
	src/main/scala/mesosphere/marathon/health/HealthCheckActor.scala
	src/main/scala/mesosphere/marathon/health/HealthCheckManager.scala
	src/test/scala/mesosphere/marathon/health/HealthCheckTest.scala
Make scheduler driver a singleton. Consecutive failures is required.
Merge pull request #291 from mesosphere/max-failures

Add support for max consecutive health check failures