Add pypcap as a scapy dependency.

On Mavericks, opening scapy without pypcap installed results in an error
message. While is seems not fatal, it looks better to me to fix the issue.

Please consider also #93 as a companion to this one, as on that same OS I can't install pypcap if Pyrex is not installed, so actually merging only this PR without #93 would break its installation.