Parsing packet timestamp and passing it to the callback as second arg.

Hello,

thank you for the implementation of the libpcap interface in Python. I've used it the last couple of days to capture some RadioTap headers, which worked fine so far.

I needed the packet's timestamp for statistics, so I implemented it to be part of the callback. The packet timestamp is converted to a double float representing the second from epoch.

I don't know if you are interested, but I thought I'd share as it was on your TODO.

Best,

Benjamin