add_to_cart_url add_to_cart_text and single_add_to_cart_text class methods allowing greater control with custom classes.

Closes #3484 and Closes #3143. Pinging @thenbrent as the handler was
originally his idea and I've killed it.

@helgatheviking In 2.1, add to cart URLs will only be used in loops, so
conditonal logic should no longer be neccessary. Single templates post
to themselves without the need for the URL.