Add the 'bind_address' configuration key.

Basic support for setting the bind address of the udp and tcp
sockets. Works with IPv4 addresses only (hostname's won't be
resolved).

Add an INFO level log message concerning the ports and addresses that
statsite is currently listening on.
Merge pull request #15 from gvalkov/bind_address

Add the 'bind_address' configuration key.
Merge tag 'upstream/0.3.5'