Make a few bits of the Olm spec a bit clearer

It would be nice if the tests tested what we'd built, not something different.

This fixes an incorrect re-declaration of ed25519_sign.

Implement header files for some of the other library bits so that we don't need
to declare functions in crypto.cpp.

something about 'global' not defined

Build into 'javascript' dir and tell package.json exactly which files we care
about.

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.

Add a basic specification for the olm protocol and format.

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

Initialise the length fields of the reader struct in decode_message, even if the message is invalid, fixes a crash where the message was too short