OgoneGateway: New :store_amount option

Hi,

Some banks don't authorize too small amounts. The new `:store_amount`
gateway option allows to specify a different amount for the authorization
part of the `#store` method. Let me know what you think! :)

Unfortunately I was not able to run the remote test. By any chance,
@ZenCocoon would you be able to run the remote tests?

Thanks!