Excluding generated code works for Checkstyle, not yet for Findbugs.

See http://stackoverflow.com/questions/22037499/using-excludes-config-in-findbugs-and-checkstyle-plugin-in-gradle
Don't show checkstyle violations directly. Removed Checkstyle classpath config, seems to be fixed.
Added PMD plugin, seems to work out of the box.
Merge branch 'master' into serializer-protobuf
Exclude for Checkstyle, Findbugs, and now PMD.