Echo (and Zero)

http://jenkins.buttonweavers.com:8080/job/buttonmen-blackshadowshade/97/

Currently, Echo vs Echo or Zero reverts to the recipe for Avis being used by both players. Changes to this behaviour should be requested in a new issue.

The extra PMD warnings will be dealt with once the new PMD settings are in place.

This depends on pull requests #535 and #542.

Fixes #213.

This requires a database update to change the recipes of Echo and Zero to ''.