Added networking support #13

Networking is handled by the NetworkHandler portion of the code, which
provides a few functions for use by other parts of the program, and a
status callback to monitor for connectivity changes. Connectivity is
also handled in an asynchronous mode, to avoid freezing the entire OS
while actions are performed.

• network_connect should be called by any part of the program wishing to
  establish connectivity
• network_disconnect should be called to disconnect the network once
  done
• network_init is a special function which should only be called by
  main. It is responsible for setting the network interface in
  asynchronous mode as well as attaching the status monitor callback. No
  other part of the program should ever call this.

Lwip is used for the network stack, and IPv6 support has been enabled,
and prefered over IPv4 if available.

Added networking support