Except for the esnext builds, which use the original code.

Fixes #866

Mostly replaced ``var`` with ``let`` and ``const``.
Also added a few arrow functions.

So that it's not necessary to set up the whole dev environment to use
index.html or inverse.html

and also use the Strophe.LogLevel levels for logging in general.

Also add an API method for exposing new promises.

Update the plugin and API documentation, specifically concerning the above
changes.

During logout we don't have enough time.

Otherwise rooms cannot be exited properly.

Also, no need to call `_tearDown` in `logOut` when `connection.disconnect` gets
called since it will get called afterwards in the handler.