redo how exceptions are handled during pcap callback
redo how exceptions are handled during pcap callback

  * turn pcap_handler_ctx into a Python class so that we can directly
    assign objects rather than cast everything to PyObject *
  * ensure that only one exception can be raised
  * save the exception info in the callback to prevent it from being
    clobbered after the callback returns

fasguard/fasguard-pcap,fasguard/fasguard-pcap