Everything is !important, social buttons, class cleanup, docs cleanup

• Potential Breaking Change: Added !important to all utility classes
• Breaking Change: .xs-show has been deprecated. Use the responsive display classes found in Utilities going forward.
• Breaking Change: .xs-half-width and .xs-full-width have been deprecated in favor of using .xs-col-6 and .xs-col-12.
• Breaking Change: .block-grid-item is now .block-grid__item, in line with our other BEM classes.
• Added specific line heights for each type size.
• Added buttons with icons and social buttons.
• .block-grid is no longer necessary to create a Block Grid.
• Added .xs-static and .xs-overflow-visible in case one needs to reset those at a specific breakpoint.
• Fixed overflow classes to be responsive.
• Rewritten Grid, Block Grid, Responsive and Utilities documentation.
• Overhauled navigation. Now with hot new sublinks and design!