Adds five HTML tags to ElementFactory to improve HTML 4.01 compliance and to be more consistent with the major layout engines:
1.) DFN - render in italics, same as EM and I. (WebKit doesn't render DFN in italics, but Gecko, Trident, and Presto do.)
2.) VAR - renders same as DFN (WebKit does italicize VAR, same as all the other major layout engines)
3.) SAMP - renders like CODE and TT in the major layout engines.
4.) KBD - same as SAMP.
5.) S - renders same as STRIKE: http://www.w3.org/TR/html401/present/graphics.html#h-15.2.1
This patch also uses the same object for "B" and "STRONG", instead of creating two separate objects using HTMLElementBuilder.Strong() (like only one object is used for the 3 tags: I, EM, and CITE.