Add IDs to `Messages`

They will be used for getting buffered messages starting after a certain
received message.
First attempt at "buffered sockets"

Need to test and such.
Fix format of code in ISOBlueLibraryDemo
Add sockets reading buffered data to demo
Make demo app distinguish old and new messages
Fix bug with `ISOBlueCommand` field `bus`

The type being used was too large,
so when it had too big a value ISOBlue got confused.
Fix `toString` method of `Message` class
Add "buffered" sockets to the README