Fix for https://github.com/m2e-code-quality/m2e-code-quality/issues/9
Switch required execution environment to Java 1.6
PMD itself needs 1.6, checkstyle plugin code needs 1.6 too
Upgrade pmd-eclipse-plugin to 4.0.0
Add test project for PMD.
Merge pull request #19 from zeilush/master

NPE Fix
merging pull request from adangel: Add support for pmd eclipse plugin 4.0.0