More verbose selftest

* src/pf.lua: Print the str in the selftest.
Import pcap file reader and writer from snabbswitch.

savefile.lua was originally snabbswitch/src/lib/pcap/pcap.lua in
snabbswitch, and was written by Luke Gorrie.  It is under the Apache 2.0
license, like pflua.
Fix readme instructions

* README.md: Update.
Add inline bounds checks for packet access

* src/pf.lua: Remove unused P helper.  Add inline bounds checks for
  packet access, as they need to cause the filter to exit prematurely.
Add packets-as-strings interface

* src/pf.lua: Add buffer implementation for strings.  Finally we can
  test the filters.
Add samples and a small savefile reader test

* src/samples/COPYRIGHT:
* src/samples/v4.pcap:
* src/samples/v6.pcap: Import from snabb.  Originally from
  http://wiki.wireshark.org/SampleCaptures.  See COPYRIGHT for details.

* src/pf.lua: Add test that the savefile reader parses the right number
  of packets.