789 arbitrary docker options

_Summary of changes:_

- [X] Added new parameters `privileged` and `parameters` to `marathon.proto`.
- [X] Regenerated `Protos.java`.
- [X] Added to `mesosphere.marathon.state.Container.Docker`.
- [X] Added serialization (JSON, protobuf) tests.
- [X] Amended docs for new options.

_Anecdotal evidence of success:_

```http
HTTP/1.1 201 Created
Content-Type: application/json
Location: http://mesos.vm:8080/v2/apps/bridged-webapp
Server: Jetty(8.y.z-SNAPSHOT)
Transfer-Encoding: chunked

{
    "args": null, 
    "backoffFactor": 1.15, 
    "backoffSeconds": 1, 
    "cmd": "python3 -m http.server 8080", 
    "constraints": [], 
    "container": {
        "docker": {
            "image": "python:3", 
            "network": "NONE", 
            "parameters": {
                "magic-word": "xyzzy"
            }, 
            "portMappings": null, 
            "privileged": true
        }, 
        "type": "DOCKER", 
        "volumes": []
    }, 
    "cpus": 0.25, 
    "dependencies": [], 
    "disk": 0.0, 
    "env": {}, 
    "executor": "", 
    "healthChecks": [], 
    "id": "/bridged-webapp", 
    "instances": 2, 
    "mem": 64.0, 
    "ports": [
        0
    ], 
    "requirePorts": false, 
    "storeUrls": [], 
    "upgradeStrategy": {
        "minimumHealthCapacity": 1.0
    }, 
    "uris": [], 
    "user": null, 
    "version": "2014-11-18T21:36:32.969Z"
}
```
Make this a function to prevent code duplication.