Convert command semantics from per-service to per-environment

[ccheetham]

Basically, migrate from:

  add-service         Add a service.
  check-service       Check the status of a service in the target environment.
  list-service-types  List available service types.
  list-services       List available services.
  list-targets        List available target environments.
  remove-service      Remove a service.
  set-target          Set the target environment.
  start-service       Start a service in the target environment.
  stop-service        Stop a service in the target environment.

to something like

  add                 Add a service.
  disable             Disable a service.
  enable              Enable a service.
  remove              Remove a service.
  status              Show status and other information of a service in the target environment.
  start               Start enabled services in the target environment.
  stop                Stop running services in the target environment.
  target              View or set the target environment.

[cf-gitbot]

We have created an issue in Pivotal Tracker to manage this:

https://www.pivotaltracker.com/story/show/160004605

The labels on this github issue will be updated when the story is started.