Graphlot - enable dynamic template selection using special URLs

This patch enables the dynamic selection of Graphlot HTML templates by changing the URL.  For example, a url like http://graphite/graphlot/embed?width=586&height=308... will use the graphlot_embed.html template (if it exists).  If the URL implies a template that does not exist, django automatically falls back to the standard graphlot.html template.