Use IE=edge by default.

This patch fixes this issue: http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=32396&start=0

It's a template issue but, rather than fix it in the template, I'm fixing it in the HTML document class. Why?
1) Easy to override (for testing or whatever) without modifying any Joomla! files.
2) Future template writers need not be concerned with remember the tag.
3) 99.9% of all templates that will ever be created should want to use this setting anyway.