Revise HD Protocol Text

Preview: http://dg2.dtrt.org/en/developer-guide#heirarchical-deterministic-key-creation

* 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.)