Added a framework message event and support for publishing these events ...

I added a framework message event and post these events to the event bus. This pull request is for issue #228.

I followed the pattern for existing events and and for the framework message class parameters I just used what is provided as part of the framework message (i.e. the executor ID, the slave ID and the message). Here's the case class:

```scala
case class FrameworkMessageEvent(
   executorID: String,
   slaveID: String,
   message: Array[Byte],
   eventType: String = "framework_message_event")
  extends MarathonEvent
```

For my own testing and inspecting events I created a simple HTTP endpoint application based on the [Play Framework](http://www.playframework.com/) which can be used to receive and log POSTed JSON data. Here's the application: https://github.com/marc-barry/HTTP-JSON-Validator.

Just run the Play! application and configure Marathon to post messages to the endpoint:

```
/bin/start --master zk://localhost:2181/mesos  --zk_hosts localhost:2181 --event_subscriber http_callback --http_endpoints http://localhost:9000/validate
```

The ```http_enpoints``` command line argument is just the endpoint of the Play! application which accepts and validates JSON.