Remove the overrides and resets from button groups

This pull request makes extensive use of the `:not()` selector to remove the redundant CSS and resets for border-radii. It also adds a `-1px` top margin to vertical input groups to match the horizontal input groups.

Unlike the input group fix, this one is little more controversial since it will break backward compatibility for vertical input groups, since you no longer want to apply the normal `.btn-group` class to vertical groups.