Unix socket support

Usecase:
Webserver and facette are running on the same machine. There is no need for a network socket. Access to unix sockets can more easily be restricted to users on the local machine.

Implementation:
As far as I can tell, the changes are backward compatible. The code can deal with missing key ins the configuration JSON and it will be switched to unix sockets if the bind address starts with an "/", witch can't be the beginning of a hostname/address.