Introducing $ie7-support variable.

This variable (set in core/_presets.scss) will indicate whether
little ol' IE7 should be supported. If false, v2 of normalize.css
will be used and styles specific to IE7 (to be changed later) will
be omitted from stylesheets.
More IE7 styles conditionally excluded.

Also using Compass hyphen mixin and @extend for hiding #jshint.