Fixes that the ruleset was ignored (fix for #21).

PMD checks the extension of the ruleset location to figure out, whether
it is a real file or not. Could be that this is new with PMD 5.
That's why the generated project ruleset must have a xml file extension.
Fixes that the ruleset java-unnecessary was ignored (fix for #21).

Now m2e-quality-pmd uses the same logic to load the rulesets as
maven-pmd-plugin does: RuleSetReferenceId.getRuleSetFileName().
This allows for the "special" built-in ruleset names like "java-basic".
Update site location for pmd.
Add workaround for PMD Eclipse plugin bug if a non-existing source folder
is in the eclipse project configuration.
See: https://sourceforge.net/p/pmd/bugs/1116/
adding pmd files to .gitignore