Add Font-Awesome support

CKEditor strips out `<i>` tags when you go from source view to preview, which means you cannot add Font-Awesome icons from within the editor without adding a `&nbsp` inside, which is problematic since the space is rendered. This pull request removes `<i>` from the list of nodes in the DTD that are marked for removal if empty, which allows `<i>` tags to remain.

It would be nice to add support for configuring DTD options in the initializer, but since you must delete an item (rather than set it's value to 0 or false) to turn it "off" things get a bit tricky, and that's a whole other can of worms that I didn't feel like getting into at the moment.