See extended description.

- Moved brand related classes into a single mixin. Now one brand class
can be used to skin any element.
- Added '.arrow--top-center'.
- Clarified comment relating to 404 error in
styles/structure/objects/_sprite.scss.
- Moved all '.mask' related properties into
styles/skin/objects/_mask.scss - to be edited later.
- Corrected padding on '.thumbnail'.
- Removed '@extend .btn' from [type="submit"]. Class should be added in
markup.
- Slightly changed '.button' and '.pill' base color styles.