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 know, that's why I left the TODO there.

> We could create a wrapper that will message the parent once the server is listening.

That's not so straightforward:
 - we have to rely on the fact that `server/server.js` exports an express app
 - whatever extra code guarded by `if (require.main === module)` will not be executed

For the reasons above, I'd rather implement an TCP or HTTP ping solution:

 - `GET /` or connect to `host:port`
 - call `done` on success
 - recurse on failure

Can we leave that for another PR, so that we can start building the UI part on top of the API added by this patch?