Add optional handlers for publisher errors, access denied, and audio level change

[sedenardi]

Right now the only error messages you get are if the test setup itself fails; all errors from the OT.Publisher are left unhandled (and just dump to the console). This PR allows the client to handle these errors to give a more accurate sense of the problem (such as whether the hardware isn't there, whether the browser is blocking it, whether the mic is muted, etc.).

I've also modified the references to window so that this can be used in node.js and webpack environments where the window variable isn't declared.

I've updated the tests and the docs accordingly. Let me know if you have any questions, comments, or concerns.