PayPal donations

**WIP**
https://github.com/metabrainz/metabrainz-server/blob/master/lib/MetaBrainz/Data/Donation.pm#L179-L182 uses `payment_gross` variable instead of `mc_gross`. [Documentation](https://developer.paypal.com/docs/classic/ipn/integration-guide/IPNandPDTVariables/) says that this is a legacy field replaced by `mc_gross`.