FirstChildElement and namespace prefixes.

[olekrisek]

The namespace prefixes should be ignored when querying for a give tag-name, as when parsing an XML, you often can't predict the namespace prefix without analysing the namespace structure. Either as an optional parameter to "FirstChildElement", or as a standard feature.

I have cloned the repo and done the changes to it. It's on https://github.com/olekrisek/tinyxml2

I can send a pull request if you are interested.