Fixed lexing of XML comments.

The previous setup would consume too much. For example the following HTML:

    <a><!--foo--><b><!--bar--></b></a>

would result in the following T_COMMENT token:

    "foo--><b><!--bar"

The new setup requires the marking of a start position. I'm not a huge fan of
this but there doesn't appear to be a way around this.
Fixed lexing of XML CDATA tags.