Wip running instances

Adds `"tasksStaged"` and `"tasksRunning"` members to the JSON representation of `AppDefinition`.

Minor housekeeping elsewhere.

Related to #124, #118, and #126

See the following example:

```
GET /v2/apps/myApp HTTP/1.1
Accept: */*
Accept-Encoding: gzip, deflate, compress
Host: localhost:8080
User-Agent: HTTPie/0.7.2

HTTP/1.1 200 OK
Content-Type: application/json
Server: Jetty(8.y.z-SNAPSHOT)
Transfer-Encoding: chunked
```
```json
{
    "app": {
        "cmd": "sleep 60", 
        "constraints": [], 
        "container": null, 
        "cpus": 0.1, 
        "env": {}, 
        "executor": "", 
        "id": "myApp", 
        "instances": 4, 
        "mem": 5.0, 
        "ports": [
            12814, 
            15793
        ], 
        "tasks": [
            {
                "host": "mesos.vm", 
                "id": "myApp_2-1391652824686", 
                "ports": [
                    31004, 
                    31005
                ], 
                "stagedAt": "2014-02-06T02:13+0000", 
                "startedAt": "2014-02-06T02:13+0000"
            }, 
            {
                "host": "mesos.vm", 
                "id": "myApp_3-1391652846712", 
                "ports": [
                    31002, 
                    31003
                ], 
                "stagedAt": "2014-02-06T02:14+0000", 
                "startedAt": null
            }, 
            {
                "host": "mesos.vm", 
                "id": "myApp_2-1391652835698", 
                "ports": [
                    31000, 
                    31001
                ], 
                "stagedAt": "2014-02-06T02:13+0000", 
                "startedAt": "2014-02-06T02:14+0000"
            }, 
            {
                "host": "mesos.vm", 
                "id": "myApp_2-1391652819678", 
                "ports": [
                    31006, 
                    31007
                ], 
                "stagedAt": "2014-02-06T02:13+0000", 
                "startedAt": "2014-02-06T02:13+0000"
            }
        ], 
        "tasksRunning": 3, 
        "tasksStaged": 1, 
        "uris": [
            "https://raw.github.com/mesosphere/marathon/master/README.md"
        ]
    }
}
```