Improved HTML5 serialiser

Improved html5 serialiser which respects the html5 syntax (as expected by most external tools):

* void elements like input, meta or img never have content and must not be closed by an end tag
* script and style are "raw text" elements whose content should not be escaped
* attributes with name=value (like checked="checked") are output as empty attributes without value

The html5 serializer will **not produce well-formed xml**. Use the xhtml5 serializer if you need the output to be well-formed.

Future todo: use the version serialisation property to distinguish between html serialisers.