The Verto libs currently have total control over the streams associated with
placing any kind of call, handling both their creation and teardown
automatically.

This patch provides the option for a developer to instead pass pre-created
MediaStream objects when instantiating the Verto object, or when calling
Verto.newCall(), and the library will bypass the work of creating those
streams, and of destroying those streams when the call is torn down.

This is particularly useful if the application wants to manage its own streams,
such as re-using them in other non-Verto aspects of the application.

The patch also creates some internal convenience functions for managing the
video element related to a local video stream.