Added @if $legacy-support-for-ie6 & @if $legacy-support-for-ie7 support

This allows you to control whether or not you want to add the IE 6
extra classes and the IE 7 css hacks to the formalize css.
Converted the updated sass file to scss

sass-convert _formalize.sass -T scss  _formalize.scss
Merge pull request #67 from alienresident/master

Added Compass @if $legacy-support-for-ie6 and @if $legacy-support-for-ie7 to _formalize.sass