Export `Message` class (and possibly other classes)

[arnoson]

I'm using your library in a project where I only need the Message class. It would be great if I could import it like this:

import { Message } from 'osc-js'

I could just import it like this, but this will load all the other code which I don't need in this case

import OSC from 'osc-js`
const { Message } = OSC