Amended Makefile to copy content of videos directory into
installation directories. Also copies videos and images
into the html/ directory during the build, so that the
built cookbook can be viewed locally (for testing without
having to install).
Added an XSLT template to transform Docbook <inlinemediaobject>
elements into HTML 5 <video> elements, with a fallback to
link to the video displayed for browsers without HTML 5 support.
Added note to "Contributing" appendix explaining how to put
video into a recipe.
