Dynamic payment modifier valid option updates

[racitup]

Payment modifier is_disabled() changes will be reflected in radio options being disabled dynamically.
This has specifically been developed to allow changes in shipping modifier to dynamically affect the available payment modifiers and is a solution to issue #592 -

Shipping modifier:

    def add_extra_cart_row(self, cart, request):
        cart.total_incomplete = False
        # if not added and others available
        if not self.is_active(cart, request) and len(cart_modifiers_pool.get_shipping_modifiers()) > 1:
            return
        ...
        cart.total_incomplete = True

Payment modifier:

    def is_disabled(self, cart):
        "if shipping cost is unknown, disable"
        disable = (cart.total == 0) or getattr(cart, 'total_incomplete')
        return disable

[jrief]

Please be patient on this pull request.
I have to check on its side-effects.

[racitup]

I've noticed the "Purchase" button no longer works on my implementation, but seems to work on the example..

[racitup]

The "Purchase" button is now working on my project too. I noticed some differences in the settings between my project and the example project. It could have been that or something else that fixed it. Possibly the addition of the fontawesome iconfont?

[racitup]

This requires some changes to work properly, for example it is possible to select a payment option radio button, change the conditions so that the radio button gets disabled yet the form still validates successfully. Ideally form validation would fail forcing the user to select a valid payment option.
I'm not sure how to fix that..

[racitup]

Now also rebased on 0.11.dev0 here

[jrief]

Feel free to remind me. There was too much stuff awaiting for 0.11