Add support for HTTP2

[acmorrow]

I see there is some SPDY work, but it isn't clear how to make it do anything. Also, since HTTP2 now supplants SPDY, maybe better to just implement that?

[mikedickey]

The SPDY parsing capability was added to support passive capture of SPDY traffic by our Stream product. It was sort of experimental; we intended to roll client/server support into the library but never ended coming back to it. As it stands now, it's basically dead code (even Stream is using something different now). It would be great if someone could rework/refactor this code as the basis for HTTP/2 support (or just replace it altogether, if necessary).