Update clearfix to use a mixin

[yodasw16]

Move the CSS for Clearfix into a mixin and then use the mixin where the CSS was before. Use the Box object as an example of where the mixin should go in the file. You will also need to update the readme so that @extend is no longer documented under "Available Classes", but instead, add it to the "Usage section".

[motionsuggests]

I was looking into this and based on correct import order making the pkg:clearfix a mixin would mean it has to be imported before pkg:box to work. Should I make another package called "tools" and put the clearfix mixin in there?

[yodasw16]

Someone already submitted a PR for this, but I haven't had a chance to look at it. #61