Hotfix: Semantic grid margins and clearfix

This PR fixes to issues with the current Semantic.gs implementation:

* Margins are now explicitly `margin-left` and `margin-right` instead of the shorthand `margin: 0 %value%;`. The former had the includes overwriting top and bottom margin every time you used it. 
* Secondly, it changes the clearfix on the `@mixin row` to be included instead of extending a silent class. Extending a silent class effectively makes `@include row()` unusable inside `@media` queries.