E2E Encryption!

The implementer's guide for E2E is now pretty much complete, and libolm and megolm is good to go. We'll hear initial feedback from the audit on Tuesday (Oct 4), but we're not expecting any showstoppers to emerge from it (hopefully!)

https://docs.google.com/document/d/17zpoQYFuE-vU5aiVpGxJi-KiMqvULP4RFMZnzyr3i6U/edit#

Obviously there's the problem that the library still talks base64 rather than binary, patches welcome there.