Work towards pep8

Signed-off-by: Simon Luijk <simon@simonluijk.com>
Add setting to redirect to shop thank you page.

Signed-off-by: Simon Luijk <simon@simonluijk.com>
Set business from PAYPAL_BUSINESS fallback on PAYPAL_RECEIVER_EMAIL

Business can be set to any email on your Paypal account. When an IPN is
received PAYPAL_RECEIVER_EMAIL is used to verify receiver_email is your
primary Paypal account email. As this could be different from the business
email we need both settings.

Signed-off-by: Simon Luijk <simon@simonluijk.com>
Set currency from shop.get_order_currency(order) if available

Signed-off-by: Simon Luijk <simon@simonluijk.com>
Add missing import

Signed-off-by: Simon Luijk <simon@simonluijk.com>
Merge pull request #9 from simonluijk/general-fixes

General fixes