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
Shouldn't this be the `WORKSPACE_DIR`?