Split P2SH And Multisig; Move Multisig To Contracts

In pull #42, @saivann proposed spliting up the P2SH And Multisig subsection and then moving the multisig subsection to Contracts.  I told him I liked the idea, but that it would require a bunch of revising---this is that revision.

Although I tried to reuse text (because I'm lazy), I ended up almost completely rewriting the text about P2SH, multisig, and escrow and arbitration.  I think this both accomplishes @saivann's goals and also
makes the overall information in those sections clearer.  It also happens to make things much more concise---almost 60 lines have been removed.

Previews:

* <http://dg3.dtrt.org/en/developer-guide#p2sh-scripts>
* <http://dg3.dtrt.org/en/developer-guide#escrow-and-arbitration>

Here's what changed:

* Add new P2SH Scripts section between P2PH Script Validation and Standard Transactions. This new section describes the custom script problem and P2SH's solution without mentioning multisig at all.

* Delete P2SH And Multisig section completly (although a small amount of text is reused in the P2SH section and the revised Escrow And Arbitration section.)

* Radically revise the Escrow And Arbitration section to (1) use a more likely scenario and (2) describe multisig while describing escrow and arbitration.

* Delete en-p2sh-multisig-workflow.svg image (with dot and png forms) as it's no longer used.

* In the Standard Transaction section, move P2SH multisig from the P2SH part to the multisig part.

@saivann suggested this change, so I won't merge until he reviews it.  I also won't merge for at least 24 hours to give other people a chance to review it.