Add Developer Examples Page / New Example Transactions

As [discussed on the docs mailing list](https://groups.google.com/d/msg/bitcoin-documentation/YHc-5L7jY74/I-UUPEIgqO4J), the developer docs are intended to have three main pages: the Developer Guide, the Developer Reference, and another page we provisionally called the Developer Cookbook (because it would be similar to the O'Reilly "[Programming Language] Cookbook"-style books.

I've added this page using the name Developer Examples.  The new page features a newly-written set of example transactions with detailed explanations, which is something that's been TODO'd for a couple months now.  It also features the extended BIP70 example from the Guide.  A preview is available here:

http://dg0.dtrt.org/en/developer-examples

The extended BIP70 example from the Guide has been removed and replaced with an [overview-level description of the Payment Protocol](http://dg0.dtrt.org/en/developer-guide#payment-protocol).

Now that we have three pages, I also gave each of the full pages their own icon on the [portal page](http://dg0.dtrt.org/en/developer-documentation).

A new CSS class, multicode, has been added to allow multiple syntax highlighted examples to be included in a single code block.  This lets us use bash highlighting for bash commands and json highlighting for JSON output, or any other combination.  It can be seen in use in many of the example transactions.

Two one-line were made to the autocrossref plugin.  The first change makes autocrossref ignore code blocks; the second change makes it remove HTML comments from autocrossref text so we can easily check to ensure it doesn't create broken links (see 61f0931b for details).

This is a large change, so I will not merge it until I receive at least one ACK.
This sentence seemed a little confusing to me on a first read. How about:

    The PaymentACK doesn't mean that Charlie's order and payment is final