Support for price/option drop-down on Buy Now button

[mikery]

Hi,

This adds support for displaying a drop-down with product/price options. The change is backwards compatible - if item_options is not set when initialising the Buy Now button form, the drop-down is not displayed.

item_options is a tuple of price/option_name pairs, e.g.:

fields = {'item_name': 'name',
    'item_options': (('10.00', 'Ten dollars'),
        ('20.00', 'Twenty dollars')
    ),
    ...
}

This change is not yet ready to commit (no docs/tests), but I would like to get some feedback on the idea/implementation. If it looks like it will get merged I will update the pull request with docs and tests.