Correct size of Bkdsa to 32 bytes (256 bits) as that it the size of the private component of the key

Signed-off-by: Daniel Thomas <drt24@cam.ac.uk>
Add support for byteconcat of (32 bit) integers

Signed-off-by: Daniel Thomas <drt24@cam.ac.uk>
Add missing return statement to NigoriNonce.toByteArray()

Signed-off-by: Daniel Thomas <drt24@cam.ac.uk>
bytes: add base64 encoding convenience functions

Signed-off-by: Daniel Thomas <drt24@cam.ac.uk>
messages: store fields as base64 encoded Strings so that JSON conversion works

Signed-off-by: Daniel Thomas <drt24@cam.ac.uk>