Latest Task Failure in App Response JSON

Adds an additional field that may be embedded into the /v2/apps response:

This new `lastTaskFailure` field is also included by default in the response JSON for `GET /v2/apps/{appId}`.

```json
{
  "lastTaskFailure": {
    "appId": "/ubuntu", 
    "host": "10.141.141.10", 
    "message": "Abnormal executor termination", 
    "state": "TASK_FAILED", 
    "taskId": "ubuntu.271abf80-5044-11e4-bd18-56847afe9799", 
    "timestamp": "2014-10-10T06:10:38.755Z", 
    "version": "2014-10-09T23:03:33.993Z"
  }
}
 ```

Or, in context:
```http
GET /v2/apps?embed=apps.failures HTTP/1.1
Accept: */*
Accept-Encoding: gzip, deflate
Host: mesos.vm:8080
User-Agent: HTTPie/0.8.0
```

```http
HTTP/1.1 200 OK
Content-Type: application/json
Server: Jetty(8.y.z-SNAPSHOT)
Transfer-Encoding: chunked

{
    "apps": [
        {
            "args": null, 
            "backoffFactor": 1.15, 
            "backoffSeconds": 1, 
            "cmd": "while sleep 10; do date -u +%T; done", 
            "constraints": [], 
            "container": {
                "docker": {
                    "image": "libmesos/ubun2", 
                    "network": null, 
                    "portMappings": null
                }, 
                "type": "DOCKER", 
                "volumes": [
                    {
                        "containerPath": "/etc/a", 
                        "hostPath": "/var/data/a", 
                        "mode": "RO"
                    }, 
                    {
                        "containerPath": "/etc/b", 
                        "hostPath": "/var/data/b", 
                        "mode": "RW"
                    }
                ]
            }, 
            "cpus": 0.5, 
            "dependencies": [], 
            "deployments": [
                {
                    "id": "f945548f-dd80-49a2-8b1a-d07ad9245688"
                }
            ], 
            "disk": 0.0, 
            "env": {}, 
            "executor": "", 
            "healthChecks": [], 
            "id": "/ubuntu", 
            "instances": 1, 
            "lastTaskFailure": {
                "appId": "/ubuntu", 
                "host": "10.141.141.10", 
                "message": "Abnormal executor termination", 
                "state": "TASK_FAILED", 
                "taskId": "ubuntu.271abf80-5044-11e4-bd18-56847afe9799", 
                "timestamp": "2014-10-10T06:10:38.755Z", 
                "version": "2014-10-09T23:03:33.993Z"
            }, 
            "mem": 512.0, 
            "ports": [
                10000
            ], 
            "requirePorts": false, 
            "storeUrls": [], 
            "tasks": [], 
            "tasksRunning": 0, 
            "tasksStaged": 0, 
            "upgradeStrategy": {
                "minimumHealthCapacity": 1.0
            }, 
            "uris": [], 
            "user": null, 
            "version": "2014-10-09T23:03:33.993Z"
        }
    ]
}
```