add type="button" to all close buttons

See https://github.com/twitter/bootstrap/issues/3465 and http://twitter.github.io/bootstrap/javascript.html#modals

This fixes an issue where pressing Enter while focusing on a text-field in a modal dialog would simply close the modal instead of submit the form as one would expect.

Since type="button" is present on all close buttons in the official docs I figured it should be fixed in this method instead of just in TbModal.