All callback urls in PGs must be relative

Some PGs use callback urls with full URL to callback. PG's are store-independent and therefore the callback url should be constructed relative to the current shop that will issue the post to PG.

In order to do this I propose to create simple service:
interface InetDomainResolver {
String getDomain();
}

There should be two implementations of this resolver:
1. For storefront that will use thread local of either ApplicationDirector, or Wicket Request to access this information
2. For YUM (this is for refunds and shipping complete captures) we need some sort of factory that will use CustomerOrder object that has getShop() and thus we can get the default url mapping from the Shop instance.