Allow custom classes on OK and Cancel buttons

- This makes it significantly easier to use vendor styles (e.g. Twitter
  Bootstrap) on those buttons

Also fixes #207.

Would you mind bumping the gem version when this pull request is accepted?  We wanted the OK and Cancel buttons to look like the rest of our app (using Bootstrap), and we had to resort to some awful SASS hackery to achieve that.  After this feature, we should be able to just specify a 'btn' class and be on our way :)