[Bootstrap 3 only] Fixed classes and elements associated with icons

The element used to display icons was `<i ... />`, which as far as I know is not a valid HTML tag. Bootstrap 3 website uses `<span>` elements for the icons in their examples now, and I made it match their practice. Also, they changed the class name/prefix from `icon` to `glyphicon` in Bootstrap 3.

This pull request will likely break the icons for websites using Bootstrap  <= 2.3.2