Implement Prometheus

Implement Prometheus

Fixes #50.

For now, it only implements the following stats:

	fort_valid_vrps_total{ta="<TA>",proto="ipv<IP>"}
		Total VRPs generated from TA <TA> (and the given
		protocol) during the previous cycle.
		"<TA>" is inferred from the TAL's file name.

	fort_rtr_current_connections
		Number of active RTR clients.

To activate the server, set --mode=server and --prometheus.port to an
allowed and available port number.

Adds libmicrohttpd as a dependency.