Move private functions and variables to internal sub-directory.
Use fully-qualified paths in @import lines.
Update comments in _variables.
Remove $zen-auto-include-box-sizing in favor of $zen-box-sizing: universal-border-box.
Add $display-zen-grid-background variable to toggle background grid between development/production builds.
Fix common syntax errors when using zen-grid-container()'s $context.
zen-grid-container() didn't properly un-do the gutter padding of the parent grid item in flow $context.
Streamline code in zen-direction-switch().
zen-grid-item() fails to use explicit params when $zen-auto-include-grid-item-base is false. Fixes #51
Rename zen-grid-item()'s $auto-include-grid-item-base parameter to $include-base. Fixes #57
Rename zen-grid-flow-item() to zen-flow-item-width(). Fixes #53
Update code comments.