Added option to restart a docker container in -notify

[JustAdam]

This solves issue #22 (not completely, but provides the requested feature). I haven't added full access to the Docker API here as I think the only reason to access it, is for restarting containers.

So to restart a container, specify -notify "restart-container CONTAINER-ID". This syntax feels a bit clunky, but adding an additional flag does as well.

[jwilder]

This is a little funky from a CLI standpoint. I have a suggested workaround that I commented on in #22.

[JustAdam]

The main purpose of this patch is too be able to run docker-gen by itself in a container. Therefore I think it is vital that this functionality is built into docker-gen and not provided by an external dependency (script/program). The external dependency provides an additional tool to maintain (ensuring the docker end point is available, ensure api calls are upto date etc.) and package.

I would really like to see this patch accepted, as it would make docker-gen much more usable (certainly for me and cnf!).

I agree with the syntax being funky, and have thought a bit more about the additional flag, and perhaps it is the only way to go. If we can agree, I will rewrite the patch to include a new flag: -restart-container=CONTAINER_ID.
So either the flag or old flag, both or none can be specified and all behaves as it did before. If both are specified then it issues the restart and the notify.

[JustAdam]

Continued in #27