Use SASS variable defaults for font path to allow override

When installing through bower or a similar package system, copying the style sheets out and modifying them to customize the path seems backwards. Thus when using the style sheets directly from the bower_components directory one needs to be able to override the font path variable. For SASS this is very easy with the ``!default`` flag, which is how Foundation solves customization too.

This adds it for all the SASS style sheets, making it very easy to import them directly from the bower_components directory and override the font path by setting a variable before importing.