Adding simple bridge application to forward calls without PBX app.

Call received on an interface can directly be forwarded to a given
destination interface, instead of routing the call through PBX
application. This way calls can be forwarded without going through
route.conf.

Currently only SIP and GSM destinations are supported. Also there
are no tones generated, if one side provides no tones, but the
other wants to receive them.

The keyword "bridge <output interface>" in interface.conf is used.
Without that keyword, incomming calls are handled as usual.