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.
Maybe it's just the way I read "descending" (which I interpret as going from the parent to the child, not going to the left node to the right node).