Added bridge module, for gathering and managing bridging informations

bridge.py works on GNU/Linux and NetBSD, very little effort is needed to port it to OpenBSD and probably FreeBSD AFAIK, unfortunately I haven't those installed anywhere on my network. In order to make this module portable to other systems, there's a dispatch-by-os function, regarding OpenBSD, it is probable that the exact same NetBSD functions should be re-used, in which case there will have to add a little trick in _os_dispatch().

The following functions are available:

- bridge.add
- bridge.delete
- bridge.addif
- bridge.delif
- bridge.find_interfaces
- bridge.interfaces
- bridge.list
- bridge.show
- bridge.stp