Upgrade Twitter Bootstrap to 2.3.1

- removed `_responsive.scss` since it is not in use (`_compass_twitter_bootstrap_responsive.scss` is used instead)
- added utility classes defined in `_mixins.scss` to `_compass_twitter_bootstrap*.scss`
- reorganized the structure of `_compass_twitter_bootstrap*.scss` files to match originals
- changed the order of imports in `_buttons.scss`