Adding a 'selected_tag' variable and fixing URLs

This 'selected_tag' variable becomes available in the page context if
the page is generated by the tagger. This enables the generated page
to display the current tag in the page body.
For example, if /news/tags/awestruct/index.html is generated, the page
can display somewhere in its body that theselected tag is: "awestruct".

Also, some URL are converted to absolute URL rather than relative 
by prepending the site.base_url variable, which automatically fixes 
navigation problems when the site is not deployed at the root context
(e.g: on GitHub)