Add support for Interface Inheritance

[oeysteinhansen]

module Example {
interface base { };
interface derived : base { }
};

I have tried to do a start. Se attatched diff.
patch_interface_20171003.txt

Not sure how this integrates with the rest of idl_parser.

[oeysteinhansen]

Here are som unittests for testing the Interface Inheritance stuff.

unittests_for_interface_inheritance.txt

[sugarsweetrobotics]

Thank you very much for your great contribution. I added implementation for interface inheritance/generalization by my hands. If you had any suggestion, please let me know.

Thank you!