Revise HD Protocol Text

* Describe the HD protocol for developers who aren't already experts in
  ECDSA key derivation.  I think this new text is a lot friendlier but
  it suffers from I-haven't-tested-it syndrome.  I'm pretty sure it's
  all correct, but I'm ok with delaying this merge until after May 15th
  if we want to keep the existing reliable BIP32 text.

* Move the old type 1/type 2 introduction to the devref.

* Delete the copied BIP32 text entirely and link to it instead. I don't
  like the idea of us maintaining an exact copy of something which
  exists authoratatively somewhere else, especially now that we have our
  own description.

* Make a few text changes to previously-made HD images. Some of the
  previously-made images are no longer used, but I haven't git rm'd them
  because I think they might be useful in the future. (Once we squash,
  we can't get them back out of revision history either.)
Merge remote-tracking branch 'saivann/dev' into hdoverview

Conflicts:
	_autocrossref.yaml
s/heirarch*/hierarch*/ (thanks @saivann!)
Remove Old Image And Move Link Definitions (thanks @saivann!)
Merge remote-tracking branch 'saivann/dev' into hdoverview

Conflicts:
	_includes/references.md
Fix one remaining "Heirarchical" typo
Fix a small typo
Merge pull request #7 from saivann/hdheirarchical

Fix one remaining "Heirarchical" typo
Merge pull request #8 from saivann/hdoverviewtypo

Fix a small typo
Merge duplicate links to BIP32
Merge pull request #9 from saivann/hdoverviewbip32

Merge duplicate links to BIP32
Use consistent jekyll syntax
Merge pull request #10 from saivann/hdoverviewsyntax

Use consistent jekyll syntax
Merge remote-tracking branch 'saivann/dev' into hdoverview

Conflicts:
	img/dev/en-hd-overview.svg
	img/dev/en-hd-private-parent-to-private-child.svg
	img/dev/en-hd-root-keys.svg
	img/dev/en-hd-tree.svg
Fix crossref links in "Wallets" subsection
Merge pull request #11 from saivann/hdoverviewlinks

Fix crossref links in "Wallets" subsection
Briefly Describe BIP39
Merge branch 'hdoverview' of https://github.com/harding/bitcoin.org into hdoverview
Add link to Wallets reference page on Developer Documentation page
Merge pull request #12 from saivann/hdoverviewdoc

Add link to Wallets reference page on Developer Documentation page