Supporting multiple pcap sessions

I refactored the pcap binding to use a class so that it can support multiple sessions (and moved some code from JS to C++ in the process).  I also improved some of the error messages.

I tested capturing on both Ubuntu 10.04 and OSX in live and offline modes, and haven't noticed any problems so far.

If you have any feedback or suggestions, let me know - I'd be happy to make any changes required.