Breaking Change - Refactors IGatewayProvider To IGatewayProviderSettings

We're *going ugly early* on this one - so apologies up front.

There has been some confusion with the naming of IGatewayProvider and it's relation to the GatewayProviderBase classes we wound up development several months later.  So we've renamed the db table from merchGatewayProvider to merchGatewayProviderSettings and patched up the Core so that it reflects the name change.