Added getter for Mesos-Stream-Id

[frankscholten]

We are using Mesos Rx Java and want to perform a kill call. Since we are not sure how a kill should be done with the Rx client we decided to use a normal REST call. However, from reading the docs on the Scheduler HTTP API it seems I require the Mesos-Stream-Id that is private in the MesosClient. This PR creates a getter for it. This is quite hacky of course so let me know what's the proper way to do this.

[BenWhitehead]

If you are wanting to send a kill for a task, you just need to publish an event in the Observable<Optional<SinkOperation<Send>>>> and the client will send that message to mesos.

If you are wanting to send the task kill not in response to an event that came from mesos, you can create a PublishSubject and merge that into the stream.

When a PublishSubject is used you can manually call onNext with the event you want to be sent to mesos.

[BenWhitehead]

@frankscholten Were you able to get things working using a PublishSubject? If so I'm going to close this.

[frankscholten]

Yes, you can close this. Thanks for getting back to me. I read up some more on Rx and I now understand how I can implement this.

[BenWhitehead]

Awesome! Thanks for checking out the library! 0.1.0 is in the process of syncing to maven central and should be available to everyone soon.