Added ability to log raw and PCAP-formatted packets in json alerts

Based on https://github.com/inliniac/suricata/pull/922 (by MattCarothers) - adds ability to log PCAP-formatted packets in JSON alerts.

Both raw and PCAP-formatted packet are logged in Base64 in "packet_pcap" field.

"How to use" information added to suricata.yaml.in.