Changed the MesosStatusUpdateEvent taskStatus parameter to a String inst...

I want to propose that the ```taskStatus``` parameter of ```MesosStatusUpdateEvent``` be changed to a ```String``` instead of an ```Int``` and its value to be changed to something more meaningful (i.e. ```TaskStatus.TaskState.name```).

Currently, the ```taskStatus``` parameter is an ```Int``` with its value from ```getNumber``` which comes from the ```com.google.protobuf.ProtocolMessageEnum``` interface. This results in JSON events like the following:

```json
{
  "slaveId" : "20140428-140502-16842879-5050-4741-0",
  "taskId" : "app_11_0-1398700942222",
  "taskStatus" : 1,
  "appId" : "app_11",
  "host" : "vagrant.myshopify.io",
  "ports" : [ 31859 ],
  "eventType" : "status_update_event"
}
```

This change would make it look like this:

```json
{
  "slaveId" : "20140428-140502-16842879-5050-4741-0",
  "taskId" : "app_11_0-1398700942222",
  "taskStatus" : "TASK_RUNNING",
  "appId" : "app_11",
  "host" : "vagrant.myshopify.io",
  "ports" : [ 31859 ],
  "eventType" : "status_update_event"
}
```

I think this provides a more meaningful set of task states such as TASK_STAGING, TASK_STARTING, TASK_RUNNING, TASK_FINISHED, TASK_FAILED, TASK_KILLED and TASK_LOST.