Basic monitoring support for Mesosphere Marathon

This is an agent check for [Mesosphere Marathon](https://github.com/mesosphere/marathon) which consumes the REST API and emits Datadog metrics.  We've been using this check against the Marathon master branch to power dashboards and it's proved to be trouble-free.

We're consuming the following endpoints (see linked doc for details):
  * [`/v2/apps`](https://github.com/mesosphere/marathon/blob/master/REST.md#get-v2apps): lists all running applications
  * [`/v2/apps/{appId}/versions`](https://github.com/mesosphere/marathon/blob/master/REST.md#get-v2appsappidversions): lists the versions of the application with id appId

See also: https://github.com/DataDog/dd-agent/pull/919 which provides Mesos monitoring.

Hopefully this saves others some time.