Support GRE interface

[Hoto-Cocoa]

I can query interfaces using this API.

However I can't manage GRE interface configuration by using this API.

Here is some example features:

• Show remote/local address of GRE tunnel when get interfaces list
• Add/Update/Delete GRE tunnel via API

[MokkaSchnalle]

This feature would be awesome. This will also fix the missing option to use gateway groups for GRE tunnels.