Option to extend rentals with either CREDITS or DEC -- not limited by the initial currency used#227

Currently, the user have to renew with the same currency that the rental contract was initiated with. This usually becomes a problem for lower league players when they use their saved up CREDITS to initiate rentals, then would want to renew with they earned DEC afterwards.

Owners should have a choice as to what currency they receive from rentals, maybe renters can pay in any currency they choose and its automaticly converted?