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

_includes/guide_wallets.md:

* Fix formula given for normal child key derivation to state that public
  keys must also be provided to the HMAC hash function. This required
  updating both text and images.

* Add one-paragraph warning about ancestor key compromise when the
  ancestor extended public key is compromised along with a descended
  private key.  Update img/dev/en-hd-private-parent-to-private-child.*
  to help illustrate this warning.

en/developer-reference.md:

* Remove %include% of previously-removed file which caused new versions
  of Jekyll to die.
Merge pull request #408 from harding/hd-ancestor-compromise

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