Set up the gradle-mvn-push helper for uploading snapshots

To be perfectly honest I have no idea what I'm doing here, but I followed the [README at the gradle-mvn-push repo](https://github.com/chrisbanes/gradle-mvn-push) and this is what I ended up with.

Supposedly all you need to do is call `gradle clean build uploadArchives`. I didn't run it in case it actually works.

If I'm completely on the wrong track, let me know or just close this.

I did notice that there's a [`deploy.gradle`](https://github.com/chrisjenx/Calligraphy/blob/master/calligraphy/deploy.gradle) script that looks to me like it was designed to be able to release `SNAPSHOT` builds, so I just filled in the variables from there.