MerchantWARE: Introduce two versions of gateway modules (v3 and v4)

It seems like old MerchantWARE accounts don't work with the new
version of API (v4) and the new ones don't work with the old version
of API (v3).

Thus ActiveMerchant contains two gateway modules for MerchantWARE.
MerchantWare uses the old API (v3) and MerchantWareVersion4 uses the
new one (v4).