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

P2P Reference Preview: http://dg0.dtrt.org/en/developer-reference#p2p-network
P2P Examples Preview: http://dg0.dtrt.org/en/developer-examples#p2p-network

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.

Note: I'm working on documenting the remaining P2P messages in a separate branch.
Oh, hmm.  I guess I conceptualized things the way the animated GIF shows them, with little arrows showing:

1. going down into the left child
2. going back up into the parent
3. going down into the right child

How about if we just s/descend into/begin processing/ in the text so it reads:

> Any time you **begin processing** a node for the first time, a flag should be
appended to the flag list. Never put a flag on the list at any other
time, except when processing is complete to pad out the flag list to a
byte boundary.

If that sounds good to you, I'll probably also see about making the same change in the parsing instructions.