Allow overriding the base form class used for Addresses via a SHOP_ADDRESS_FORM setting

Perhaps there is a better or cleaner way of doing this, but I couldn't find one, and all I needed was to make the State field a django.contrib.localflavor.us.forms.USStateField.  I didn't want to override the actual address model because I didn't need to change anything about the model itself, just the form, and I didn't really feel like hacking the template up when it can be done so cleanly from the form.  I also thought this might be helpful in situations where people **are** using custom address models and want more control over the form used to render them.