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.

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.
Add break between standard tx types and non-standard description (thanks @instagibbs!)
Fix wrong use of OP_0 in multisig and better explain why it's needed (thanks @instagibbs!)
Fix spelling mistake (thanks @saivann!)
Merge pull request #67 from harding/p2shedit

Split P2SH And Multisig; Move Multisig To Contracts