Refactor individual integrations in one single package

[r-brown]

Current integrations implementation (e.g. MyCommerce) is distributed across several packages, which increases significantly the maintenance effort. Following need to be considered:

• introduce the foundation (base) gateway classes
• move integration of specific classes in the dedicated package
  • every new implementation should be using the same pattern/structure
• avoid multiple configurations locations while enabling the new integration

Expected: a plug-in approach for enabling the new integrations.

[yushkevich]

MyCommerce was encapsulated.