Fix a buffer bounds check when decoding group messages

Add a README for the fuzzers

Fixes a segfault when a group message had exactly the length of the mac +
signature.

Also tweak skipping of unknown tags to avoid an extra trip around the loop.

These missing signatures were causing OSX to truncate 64-bit pointers
to 32-bit pointers when calling the missing methods, causing segfaults

Signed-off-by: Emmanuel Gil Peyrot <linkmauve@linkmauve.fr>

Split ratchet algorithm out to a separate section.

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

Fix jenkins.sh

Update comment on olm_account_one_time_keys

Document what it actually returns.

Remove the messsage index from olm_init_inbound_group_session

Add a olm_inbound_group_session_id method

Use the ed25519 public key as the group session id.

Some clients expect the session id to be globally unique,
so allowing the end devices to pick the session id will cause
problems.

Include the current ratchet index with the initial keys, this decreases
the risk that the client will supply the wrong index causing problems.

Sign the initial keys with the ratchet ed25519 key, this reduces the
risk of a client claiming a session that they didn't create.