Update menu_icon Method to Work With WP 3.8

WP 3.8 changed the admin icon system. The original admin sprite is still there at wp-admin/images/menu.png but the base css that applies it as a background image is gone. This patch allows us to continue to use `$books->menu_icon()` in 3.8 (vs setting it via the second array argument). 

I've added a version check so that users on <3.8 can still use the original icon system from this class. I haven't tested this on <3.8.

Usage:
```
$books->menu_icon('dashicons-admin-page');
```
or any icon classname from http://melchoyce.github.io/dashicons/