LoopWithCallback func as a (potentially) zerocopy alternative to NextEx2. NewPacket2 now supports packets in DLT_LINUX_SLL format, in addition to DLT_EN10MB, and has better error checking. NewPacket2 returns *TcpPacket instead of TcpPacket, avoiding some garbage when used from LoopWithCallback.
Merge pull request #16 from VividCortex/967-sniffer-zerocopy

* NewPacket2 supported DLT_EN10MB packets, and DLT_LINUX_SLL seemed to work most of the time... 95% of the time in a test system, with 5% packet loss in live tests.
* LoopWithCallback as an alternative to NextEx2. It enables the zerocopy path of libpcap 1.5.0+, even if it's a little more cumbersome to use.