switch from /usr/bin/python to /usr/bin/env python. this doesn't help folks whose python path points at python3 (e.g. Arch linux) though, but I see no choice than they have to change the shebangs, as we do on Synapse.  For instance, OSX doesn't have a python2 symlink, otherwise we'd use /usr/bin/env python2 shebang.

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