Dev Docs: Add P2P Messages That Request Or Reply With Data

Adds to the devel reference page detailed documentation on the following
messages: block, getblocks, getdata, getheaders, headers, inv, mempool,
merkleblock, notfound, and tx.

Adds to the devel examples page an example of requesting and parsing a
merkleblock message.

Adds to the devel docs overview pages links to the above two new
P2P sections.

Tweaks the autocrossref plugin ignore pattern to not crossref in the
middle of a GIF image name; this allows the inclusion of animated GIFs.
Format "protocol versions" table content to improve readability
Dev Docs: Tweak Revised Protocol Versions Table Style
Dev Docs: Fix Typos/Confusions In P2P Data Messages As Reported By @saivann (Thanks!)
Merge branch 'data-messages' (pull #642)
Dev Docs: Fix Broken BitcoinJ Link

The makefile tests caught this, but it looks like I broke it back in
4553439e. Strangely, I did a full build before merging that commit---and
several more full builds since---and this is the first I'm noticing the
error. I'm fixing the broken link now; I'll look into the tests later.