add a simple pcap file writer
pcap_writer: use PPI
use PcapWriter to log dataplane traffic

When we write a logfile we'll also write a pcap file with the extension
replaced by ".pcap". If per-test logging is enabled we'll open a new pcap file
for each test.
Merge into master from pull request #68:
built-in pcap captures (https://github.com/floodlight/oftest/pull/68)