Add/Improve Images

Block Chain Section:

* Simplify block chain overview image. New image looks much nicer, which
  is important as it's the first image new readers will see.

Transactions Section:

* Split complicated complete P2PH and P2SH workflow images into several
  partial workflow images, removing ambiguity and improving appearance.

* Replaced workflow images have been deleted in .dot and .svg form. PNG
  form not deleted to reduce complications in planned
  pre-bitcoin.org-pull squash.

Wallets Section:

* Create images illustrating key derivation, replacing and expanding on
  the stock BIP32 image. (Shout-out to @wink for his easy-to-read Ruby
  BIP32 implementation and excellent README.md, which clarified several
  points for me: https://github.com/wink/money-tree.)

* New images don't exactly match current text, but we're planning on
  moving the technical parts of the BIP32 text to the devref, so I'll
  rewrite them then.
Merge pull request #102 from harding/imgupdate

Add/Improve Images