Gateways re-factoring + new gateway InterKassa good for post-USSR countries.
Merge branch 'development' of https://github.com/UsabilityDynamics/wp-invoice into ud-development

Conflicts:
	core/gateways/class_wpi_authorize.php
	core/gateways/class_wpi_googlecheckout.php
	core/gateways/class_wpi_paypal.php
	core/gateways/class_wpi_stripe.php
	core/wpi_gateway_base.php
Merge pull request #24 from antonkorotkov/development

Gateways refactoring + new gateway