Comment on meaning of as_payment_method
Use quoted url tags (required in Django >= 1.5)
Freeze basket on redirect and thaw on cancel
Submit the frozen basket within success view

This involves some trickyness with the templates as the default
preview.html wants to render the open basket (which is injected into the
template context via the basket middleware).  We duplicate some of the
basket template so we can render an alternative basket (the frozen one).
Add redirect for when basket can't be look up
Adjust tests to handle frozen baskets
Update README re 0.5
Bump version to 0.5