Replace hard coded references to the 32-byte key length with a constant, add utilities for copying data to and from fixed sized arrays

Add versions of olm_session_create_inbound and olm_session_matches_inbound which take the curve25519 identity key of the remote device we think the message is from as an additional argument

Clear the random buffers once they've been used. Add methods for clearing the account and session object once they've been used

Output simpler JSON for the account keys, don't sign the JSON but instead provide a olm_account_sign method so that the user of the library can sign the JSON themselves

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