* New CSS component – Button Group
 * Added support for stacking buttons within `<div class="button-group">` in either horizontal or vertical fashion
 * Added support for horizontally stacked responsive button groups
* Corrected an issue where billboard component was consuming CSS `background` property to set background color, instead of `background-color`
* Extracted a button border radius setting into `_variables.scss` and changed the value to `5px` (it was `4px`)