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.