Implementation some APIs of the Marathon and error decoder

I think this library is very useful for marathon users (including me).
So I'd like to go on with contribution to this.
I have implemented some APIs below.
1. POST /v2/groups
2. DELETE /v2/groups/{groupId}
3. GET /v2/groups/{groupId}
4. GET /v2/deployments
5. DELETE /v2/deployments/{deploymentId}

and added an error decoder for http error responses.
MarathonException throws when
1. request app information which not existed
2. create existing app (duplicate or conflict)
3. delete not-existed app
4. request group information which not existed
5. create existing group
6. delete not-existed group

About my some commits,
I've modified some codes for docker container and return value.

Please let me know if you have problems with this commits.
Can you please remove the commented code ?