* Add IEGeneric node type handling.
* Remove dependency on plugin_api library.
IEGeneric type name is passed via Visiotr API as new syntetic atribute.
* Add custom operations support
* Fix charachter literals comparison.
* Pass custom opsets to CNNNetwork:serialize().
IE extensions are stored in ngraph CNNNetwork and later used to pass
custom opsets to serialization transformation.
* Refactor custom ops tests to use template_extension library.
* Add comment on __generic_ie_type__ purpose.