BUG: ampersand escaping

I followed chillu hint on https://github.com/silverstripe/sapphire/pull/1264

I used Convert::raw2xml() in Requirements->includeInHTML(), and tested it on SS with both XHTML and HTML5 templates. I also modified the test suite in order to validate with it. Here you are W3C a validation POC (just html files, no SS):

* HTML5: http://lab.zirak.it/html5.html
* XHTML 1.0 strict: http://lab.zirak.it/xhtml.html
* XHTML 1.1: http://lab.zirak.it/xhtml11.html