Add support to TinyMCE 4 in WordPress 3.9

This fix the support with TinyMCE 4 in WordPress 3.9.
I know we'll totally drop this button in WooCommerce 2.2, so I sent this patch in 2.1.

The users will have more time to adjust to the change and we have time to create a plugin for the shortcodes.

This will make some people happy in #5130