Workspace: implement start/stop/restart

REST API:

    > POST /api/workspaces/start
    < { pid: 1234 }

    > POST /api/workspaces/stop
    < { exitCode: 127 }

    > POST /api/workspaces/restart
    < { pid: 1235 }

Close #127 

/to @ritch please review
I'll use `object` for now. 

For the future, we should implement a way how to define remoting data structures like `StartResult` or `FindFilter` (for DAO method `find`) at the remoting level. Note that these entities are not LoopBack models, as they don't have any behaviour.

The benefit of using such named types over anonymous `object` is to provide more information to the clients, code and documentation generators. If we had `FindFilter`, then the loopback-sdk-angualr could detect parameters of this type and include more useful documentation in the generated ngdoc.