Enhancements and bug fixes

 * Report start and end times (also parse them, assuming they are UNIX timestamps)
 * Report total capture time
 * Report total capture speed in Kbps
 * Report total bytes captured (and in megabytes, and in kilobytes)
 * Sanity check 'caplen' field incase packets are truncated to an insanely short length (seems unlikely)
 * Rename variables and structures for clarity