Support for pcap_create and associated functions

Following the discussion on [issue 1](https://github.com/maddingue/Net-Pcap/issues/1), here is the pull request with the patch discussed with some tests added.

Functions added:
  * pcap_create(): create a non activated pcap handler
  * pcap_set_buffer_size(): set the capture buffer size on a non
    activated pcap handler
  * pcap_set_promisc(): set the posmiscious mode on a non
    activated pcap handler
  * pcap_set_snaplen(): set the snaplen on a non activated pcap handler
  * pcap_set_timeout(): set the capture timeout on a non activated pcap
    handler
  * pcap_activate(): activate a pcap handler created with pcap_create()