Gas refund from lock

[julien51]

We want the locks to be able to refund gas to the msg.sender of the transaction.
Right now any address can purchase a membership on behalf of someone else (because the purchase function has a beneficiary address.
We need to add the ability for the lock owner to set a percentage of the key price to be refunded to the sender. By default it should be 0.

This will enable better key purchases through credit cards, as well as cross-chain transactions.

[julien51]

Let's actually change the approach and use a fixed value for the gas refund.