add more information to Clover's xml reports

see support case https://support.atlassian.com/browse/CLV-5734

We could pretty easily add visibility="public|package|protected|private" attribute to <line type="method" ... > elements, which would allow the user to ignore methods of certain visibility. I think the addition of this attribute would cause minimal disruption to existing consumers of clover.xml. Note that clover.xml doesn't group statements by method, so this wouldn't allow statement coverage to be filtered by method visibility. You would only be able to calculate method-entry coverage for a given visibility.