Support non-recurring contributions

Created by: wpf500

I've done some testing with memberships around when ContributionRecur records exist, and it seems that its only when the membership is set to auto-renew, as @artfulrobot suggested.

The plugin currently creates GoCardless subscriptions and not payments (which are one off), so does not support non-recurring contributions. I'm hoping I can get that functionality working at some point, but for now, just leaving this issue here as a reference. Just in case anyone is worried, any non-recurring contributions won't get silently turned into recurring ones, the plugin just throws an exception.