Dynamic column grid background image

Sometimes during development it's nice to have a visible grid, so this feature adds that ability.  It will adjust the size of column and gutter gradients to match the settings for the grid.  There are a couple of quirks in the code that are solutions to issues I ran into.  Specifically:

* When applying to a row, the padding on the row below document max-width was throwing off the calculations of the background position and size, so a wrapping element is inserted with jQuery.
* The oneColAndGutter function needs percentage to properly calculate the size for Safari; not sure why.
* The Compass mixin for gradients doesn't work properly in IE10, so this feature only works for Chrome, Safari, Firefox (and potentially other browsers I haven't tested).  It also requires JS due to the above restriction.