DNN-5279 Add overloads in XmlUtils that take XPathNavigator

Since XPathNavigator is more general, it should be preferred in an API over
the concrete XmlNode (i.e. if I have an XPathNavigator instance, I can't use
these methods, but I can trivially convert an XmlNode into an XPathNavigator,
if necessary)
Merge pull request #100 from bdukes/XmlUtils

DNN-5279 Add overloads in XmlUtils that take XPathNavigator