A cleaner IE7 implementation

I don't like the idea of having another mixin I would need to call in addition to grid-widths to set up for IE7 and below.

Following the practice of other mixins such as the media mixins ($no-mq), I would like to propose a new variable that will exist in the IE7 base stylus file. When true, the mixin will create the expressions for the widths in the IE7 file rather the the percentage widths normally created.