Added new template variables isdiscusspage, articleUrl and discussionUrl.

This makes it possible to treat discussion pages specially in the templates, and to have links between the article page and the discussion page.
Added template variables articlename, discussionname and isarticlepage.

This is just for consistency and completeness.
Merge pull request #403 from duairc/discussionUrl

Added new template variables isdiscusspage, articleUrl and discussionUrl.