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.
Given that one may need to get the hashes of additional childs, would it be more accurate to say the following?

The hash needs to be computed. Process the child nodes to get its hash, starting with the left child node. Once you have the hashes of the two child nodes, concatenate the two hashes as 64 raw bytes and hash them to get this node’s hash.