add xml and innerXML for RXMLElement, which returns the XML content of an element and its children.
Fix bug for not returning text of  child TEXT node
rename methods to make them more consistent
The innerXML should work on all type of nodes, instead of just TEXT and ELEMENT. Fix CDATA node innerXML not proper.
Merge pull request #35 from siuying/innerXML

Add innerXml and xml