Add States / Provice selection for US and Canada

[pippinsplugins]

When a user chooses either US or Canada in the country selection form, the State / Provinces options need to be added with ajax.

[evertiro]

Don't have anything that allows on-site credit transactions so I can't test it, but the commit is based on a similar function I used in another plugin, so it should work.

[pippinsplugins]

Looking good, except the JS will need to be moved to form-validation.js. edd-ajax.js is only loaded if ajax is enabled, and we would want this to function at all times.

[pippinsplugins]

This has all been finished: https://github.com/pippinsplugins/Easy-Digital-Downloads/pull/303

Some updates will need to be made to gateways that use CC billing fields (such as PayPal Pro), but these updates will make the gateways much more light weight now, since there really won't be a need to create their own CC fields.