MerchantWARE: Add reference purchase and convert all calls to v4

I think old version of API (v3) of MerchantWARE doesn't work for new accounts. At least I can't run remote tests using my test account.

This pull request converts all API calls to MerchantWARE to use new v4 version. It also adds reference purchase (using transaction id of prior transaction to make a purchase). MerchantWARE calls this 'RepeatSale'.

The only incompatible change is removing of 'credit' method. As far as I can see MerchantWARE doesn't have it in the new api version. It was marked as deprecated in ActiveMerchant also, so I've just removed it.