Integration of pflua and Snabb Switch

* Updated pflua.
* Fix minor thing in deps/pflua/Makefile (use bash as shell).
* Added a new packet filter application using pflua (based in the current Snabb Switch's packet filter application).
* Add flag to ./snabb to use pflua (harcoded to true for the moment). The flag is needed to avoid including the pcap headers twice (ffi.cdata returns an error if the same C types are declared twice). This flag could also be used to pick what filter to use (libpcap based or pflua based).
* Modified filter.lua to use pflua instead of libpcap.
* I'm using the bytecode pipeline because operator 'icmp6' was not implemented in the pflang pipeline. I will try to use the pflang pipeline.
* I added some selftest examples to applications using filters. The selftest verifies the filter is working fine.