Feature - validation of sitemap against schema

Thank you for your library - I have found it very useful in my current project. In my previous life I had to deal with the Java XML and XSD libraries and so I have added a `validate-sitemap` function. I have not changed your API other than to make UTF-8 encoding explicit when writing out the XML file. There is example code in the README file, and I have added some syntax-highlighting markup to it also.  

NB I *have* tried to squash my commits but it hasn't worked too well and I don't want to risk breaking everything!