Add Portfolio Icon to WordPress 3.8

This solution adds the the `dashicon-portfolio` class to the `menu_icon` like explained in [WordPress core](https://github.com/WordPress/WordPress/blob/master/wp-includes/post.php#L1134)

The first css removes the background image link if it has the class `dashicons` and hides the invalid image in the older versions of WordPress.
`<img src="http://dashicons-portfolio" alt="">`