XPathCheck namespace fix + test

Tests for severity(though ignored) and xpath-namespace
Fix for xpathCheck namespaces

For testing we can't use the Main-class, since this is a Command Line Wrapper, which ends with a System.exit(), something JUnit (and because of that Surefire too) doesn't like.
Although I don't like to use an array for namespaces, this is probably the best solution with regards to the checkstyle options.
The severity-bug is fixed when removing the 
  checkContext.add("severity", getSeverity());
from XmlTreeWalker#finishLocalSetup()
but I noticed this was copied from the TreeWalker, and that seems to work. Weird...