I'm working on something like this with my game. I'll post back if I ever get it working. I just went trough the Google play tutorial making the sha1 key thing and everything done now that it's "ready to publish" in Google Play I can concentrate on making it work. I'm making a bit of progress with help of many tutorials in here. I hope the following helps someone as it helped me in my process.
Some of these tutorials may have redundant information but still I recommend using all of them as some explain some aspects better than others.
App key thing
Google play version of a similar tutorial
Android developers process tutorial
Almost like in WOW the Sha1 is a pain