Add tcpdump

This pull request adds an interface to the subprocess module that lets us interface with tcpdump. Basically, I made a parser for the command line output of tcpdump and use that to set attributes on the parser class.

This code can be used to start and stop tcpdump as well as apply a filter at the end to get just the desired traffic and output the base64 encoded pcap.

@rpanah, would you please review this?
Are these the only error messages? Will these work across different platforms/versions?