Use buttons instead of links for zoom buttons

Using an `a` tag without a `href` or even with a `href="#"` can produce undesirable results in IE9, i.e. linking or triggering a route. This issue is easily avoidable by using a `button` instead. This issue is probably most relevant to single-page web applications.

I've pushed a demo of these changes to my gh-pages branch which you can see here:
http://richard-flosi.github.io/jquery-cropbox/

This Pull Request simply changes the `a` tags to `button` and updates the css accordingly to make sure the +/- are centered in the button.