Remove YUI grid

It's not currently used anywhere and we could do with a cleaner grid
implementation.
Add a simpler grid system

This replaces the YUI module we had before and implements only the parts
of the system that we need at the moment leaving a far cleaner and more
readable api.
Use floats in the grid module

This works around the nasty hacks involved in removing the whitespace
between inline-block elements without having to use comments or
negative letter/word spacing.

This does mean we can't do centered or right aligned grids very easily
at the moment. If we need these things in future we can reconsider.
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
Merge pull request #1577 from hypothesis/grid-system

Add a new simpler grid system.