Basic bill only functionality (6.x-1.x)

Hi Matt,

did a proof of concept recently where I needed to work on integrating GoCardless as an Ubercart payment provider.
Did a number of fixes and got the module to react to the GoCardless webhook requests for one-off bills only.

I will open a d.o. issue with a patch later on as well. For now GitHub works much better ;)

Cheers,
Dan