Added abdication of leadership on disconnect from the Mesos master (plus other leadership defeats/elections rework)

This PR includes quite a few things.

1. It includes the abdication of leadership on disconnect from the Mesos master. Thus, when the ```Scheduler``` calls ```disconnected()``` Marathon no longer commits suicide. Instead, it abdicates its leadership position. It is possible that this Marathon instance is re-elected which basically translates into "Wait until the master comes back." But under normal circumstances a different Marathon instance will be elected.
2. I reworked the ```MarathonSchedulerService``` class to properly handle leadership defeats and elections. 
As I learnt that ```MesosSchedulerDriver``` objects cannot be "restarted". Once one calls ```stop()``` on the driver it cannot be started again via ```join()```, ```run()``` or ```start()```. Thus, I instantiate a new driver after every ```stop()```. This insures that a new driver is ready and waiting to be started the instance of Marathon is elected.

I added comments in the code to help with understanding.

Note that I am using the term "leadership" loosely as Marathon can be elected as leader via Zookeeper or by default in the case of no HA. Basically, with my changes a single non-HA configuration will result in the Marathon instance assuming leadership from its internal perspective.