Foundation Media Queries added to global _vars.scss

Related: https://github.com/waynestate/styleguide/issues/1

@nickdenardis @robertvrabel 

This now includes the Foundation Media Queries.

I included the $rem-base which is the default base used to calculate the ems from pxs

We never define anything larger than large, so I left the xlarge and xxlarge if we ever want to use them.

If we need to override these for a site, we can do so in the local site _vars.scss, otherwise use as normal.

Usage:
```scss
@media #{$small-up} { }
@media #{$small-only} { }

@media #{$medium-up} { }
@media #{$medium-only} { }

@media #{$large-up} { }
@media #{$large-only} { }
```