Feature: new utility class "DocumentUtil"

[yelga]

• create new class 'DocumentUtil' to provide a reusable DocumentBuilder cleaner code

• Edit XmlUtil method 'stringToNode' to use the DocumentBuilder from DocumentUtil;

• Add new method 'getNodesByXpath' to find nodes within a provided XML or XSL Document using a provided XPath expression;

• Add new method 'fileHasNode' from future XSLT debugger functionality for reusability; Remove deprecated method;