Added Mandatory Flag

I added a --mandatory flag for HockeyApp to fix this issue: https://github.com/mattt/shenzhen/issues/33 (which I created).

One curious note, the mandatory flag doesn't work unless you also supply an app ID. I believe this is a bug in the Hockey API, not Shenzhen.

It might be nice to put a warning in if the user supplies --mandatory but does not submit an app ID. Let me know if you'd like me to do this.

Thanks for all your work!