Use CSS3 Flexbox in Grids when possible

This change from @msweeney makes Grids use CSS3 Flexbox when possible in order to avoid the side-effects of setting a negative `letter-spacing`. The `letter-spacing` approach is used as a fallback for older browsers, and the Flexbox rules trump it.

This should improve support for Grids across an _even larger_ set of OS/browser combinations, and its ability to withstand the use of non-default fonts when set by either the person in their browser settings or by the developer using custom fonts.

Relates to: #41, #162, #166, #189