Minor cleanup of TSMessage

I made minor changes to TSMessage to clarify the API and make the properties readonly. I don't think you should be able to change the content or the recipient of a message after it was created. Also, I think there were two bugs in TSAxolotlRatchet.mm so I put some TODOs there as I wasn't sure... have a look.

I would like to rename recipientId, contactId and [TSKeyManager getUsernameToken] to clarify that they're just phone numbers. The fact that they were called Ids when they're actually something much simpler confused me for a while... is that fine ?