Modified gutter behavior:

- Replaced padding with negative margin on element + padding on children. Base on Proportional Grids by Boon (see: http://builtbyboon.com/blog/proportional-grids)
- Added several flavors similar to other layout helpers
- Used layout mixins to allow processing of $gutter in one location (say creating a rem fallback)

Advantages: Nested guttered rows can remain flush with edge*, and can be padded like any other element. Old behavior can be reproduced by adding padding.
Disadvantages: Requires a negative left margin and width: auto. Padding among children interferes.

*miswrote in commit