Added GPG signing maven plugin
Added GPG signing maven plugin

Summary: Added the GPG signing maven plugin because GPG signed artifacts are required to perform a non-snapshot release on sonatype targetting maven central.

Test Plan:
- [Internal build](http://jenkinsmaster.mediamiser.internal:8080/view/Mercury%20libraries/job/mercury-avogadro-chris/17/)
- Ran `mvn verify` and verified that the output contained the required signature files

Reviewers: simon.baribeau

Reviewed By: simon.baribeau

Differential Revision: http://phabricator.mediamiser.internal/D2758

MediaMiser/avogadro