A couple of minor API changes

While I was updating the spec, I noticed a couple of other things I think
would be good to change:

1. AudioListener objects must now be created explicitly, but it seems better
to simply have a single AudioListener which is an attribute of the
AudioContext.
This is something I've already discussed at Apple.  AudioPannerNode objects
would now automatically have access to this AudioListener and would not need
to be set explicitly.

2. I would like to change the name of AudioOutputNode to
AudioDestinationNode. The naming seems more symmetric with AudioSourceNode:
source - destination
input - output

Also, I'm finding that there's an overload of the term "output" in the
current naming scheme, as we're referring to *inputs* and *outputs* of
particular AudioNodes when we're making connections between them.


Unless somebody has some good objections, I'll make these changes in the
document.

Chris