Dev Docs: Describe Almost All Remaining P2P Network Messages

Preview: http://dg0.dtrt.org/en/developer-reference#control-messages

* Describes all remaining messages except filterload. Specificially, the following messages are described: Addr, Alert, FilterAdd, FilterClear, GetAddr, Ping, Pong, Reject, VerAck, and Version

* New makefile test: report broken markdown tables

* Put relevant protocol version changes at the top of all message subsections.

Notes: describing filterload is next on my todo list (it's a bit more complicated than the other messages in this pull). This pull does not describe Bitcoin Core's Tor-compatible features; I don't know much about Tor and learning about it is low on my todo list.
@harding There is really a 20 minutes timeout? This one seemed surprising to me.