Fix constraint serialization
Added new task search, scale endpoints.

Now you can scale apps by killing individual tasks.  You can use the
'/v1/apps/$appId/tasks' to list tasks for an app, and then scale
apps by killing individual tasks with a POST to
'/v1/apps/$appId/scale?appId=id&host=host&id=taskid'.
Added task reconciliation.
On shutdown, don't call stop() on Mesos driver.