Specify font stacks for grids and grid units

By setting _specific_ font stacks grids, it can be assured that the
letter and word spacing will be applied correctly. The font stack for
grid units is then set back to `sans-serif`, which is also the default
font stack that Normalize.css applied to the `body`.

Closes #41
Update font-stack with recent findings

Based on conversations in https://github.com/yui/pure/issues/41/,
this font stack was updated with more fonts to fully cover all
target environments where grids were breaking.
update HISTORY.md with #41 fix
Merge pull request #2 from tilomitra/grids-fonts

Update font-stack with recent findings
Merge branch 'grids-fonts'

Conflicts:
	HISTORY.md
De-cruft and improve Responsive Grids manual test page