Add ability to inherit attributes from services

https://github.com/docker/fig/issues/610

Just initial code for discussion.

Currently, attributes 'command' 'ports' and 'name' won't be copied over to child.

example fig.yml to use

````
serviceA:
  image: ubuntu:14.04
  volumes:
    - /serviceA:/serviceA
  environment:
    PROJECT: services
    SERVICE: serviceA
  command: /bin/sh -c "while true; do echo $PROJECT; echo $SERVICE; echo ServiceA $(date) > /serviceA/date; sleep 1; done"

serviceB:
  inherits: serviceA
  volumes:
    - /serviceB:/serviceB
  environment:
    SERVICE: serviceB
  command: /bin/sh -c "while true; do echo $PROJECT; echo $SERVICE; echo $(cat /serviceA/date); echo ServiceB $(date) > /serviceB/date; sleep 1;  done"

serviceC:
  inherits: serviceB
  command: /bin/sh -c "while true; do echo $PROJECT; echo $SERVICE; echo $(cat /serviceA/date); echo $(cat /serviceB/date); sleep 1;  done"
````