Allow Getting and Setting of Inner XML Text

I noticed when creating a settings file in xml and using the XML api, that there was no way to retrieve the inner text when a node was in the form.

    <node>text</node> 

This adds that functionality as well as adding the ability to create an XMLElement in that format. It only adds getting for Lua though since unlike angelscript it doesn't seem to have the ability to use the Set functions for XML. Functionality works like follows:

    String text = root.GetChild("node").GetValue();
    root.CreateChild("node").SetValue("text");

There was one possible change I was toying with in the Set function in that perhaps it should ensure that no other child nodes have been added before allowing the call to succeed.