Replace url() with image-url().
Remove the word "default" from a variable description.
Require breakpoint.
Move functions into functions partial. Fixes #58
Update browser support variables to be compatible with Compass 1.0. Fixes #49
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.
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
Ensure version numbers are strings in calls to zen-support-for() to be compatible with support-legacy-browser().