Update _mixins.scss

EM based mixins now have an extra parameter to adjust the default font size, useful for keeping measurements on the same ratio.

Added some extra mixins.
Update _defaults.scss

Change default font size to pixels. This might fix weirdness on Macs.
Update _defaults.scss

Using a variable
Update _mixins.scss

Using variable
Merge pull request #7 from nessthehero/master

Update _mixins.scss