Add column using thirds to the pattern library
Allow a grid to be created without prefix
Use !default for the grid gutter
Switch back to inline-block for the grid system

This fixes an issue with floated columns failing to clear each other
when of different heights. We now use a flexbox implementation taken
from the Pure CSS grid system which fixes the whitespace inconsistencies
in Safari.
Pass the grid gutter into the grid mixins
Use numeric column names over descriptive ones