added better subprocess interface

added subprocess interface that parses stdout to set attributes on
the object so that other Python programs know the state of the
current command. Also enables timeouts on commands (sends signal to
kill process at end of timeout)
added tcpdump primitive

added tcpdump primitive that interfaces with the tcpdump process
via the command module. This module implements a parser for some
of tcpdump's command line output and allows for the application of
simple post processing filters after the capture is complete (this
is advised to reduce PII)
updated for pep compliance
Merge pull request #101 from ben-jones/add_tcpdump

Add tcpdump