Initial Snabb Switch Reference Manual

I am admittedly a tiny bit proud to present to you.... a fairly complete Snabb Switch Reference Manual!

By "fairly complete" I mean most parts of the Snabb Switch distribution are documented, and it should already be a useful resource for Snabb Switch hackers, old and new.

What's still missing:

* I did not document the ipv6.ipv6 app. It looks like a stub/example to me? Uses none of the protocol libraries and as far as I can tell it mostly responds to pings. Not sure.
* Not all libraries (src/lib) are documented yet. If you think a specific library should be in the documentation, let me know!
* I probably forgot *something*: You can either add it yourself or tell me to do it.
* Proof reading is highly appreciated! Don't attempt to read the whole thing at once please, might be dry. ;)

Merry Christmas!
I guess it's fairly evident that the input/output file is expected to be in pcap format given the name of the app, but perhaps it should be pointed out!?