We're standardising on 'one-time keys' as a term for the thing that Bob uploads
for prekey messages.

Split ratchet algorithm out to a separate section.

Also clean up some phrasing and correct a typo or two.

* We only use 8-byte MACs
* The version byte is \x03, not \x01

Make a few bits of the Olm spec a bit clearer

Rename axolotlpp as olm to avoid confusion with Axolotl-the-spec and Axolotl-the-OWS-libraries at moxie's request