Fix Images For Normal HD Key Derivation; Mention Ancestor Key Risk

While reading an excellent article by @vbuterin about HD wallet risks, I realized I used an incorrect key derivation formula that failed to include parent public keys in the hash. This patch for #393 fixes that error in both text and images.

I also added a short paragraph describing the risk explained in @vbuterin's article and updated the corresponding image to illustrate that risk. I would've preferred to add this new content separately after #393 gets merged, but I saw no point in updating the same image twice---once to correct my error and once to illustrate the additional risk.

Unrelated change: I removed an includes line from devref referring to a previously-deleted file; an upgraded version of Jekyll was dying because of this line.