Added standard colors for stylesheet definition, plus visual emphasis of unparsed text.

These colors use HTML colors names (http://en.wikipedia.org/wiki/Web_colors), along with the aliases.

Unparsed text handling:

- highlighting HTML output in Mode now handles unparsed text (the visual result assumes the unparsed text is always at the end, but this is not the case, and anyway you will see all the text, maybe with a very weird highlighting sometimes)
- the Stylesheet class also handles it (even if it is a bad design, see  ariatemplates/editor-backend#6)