Fix bug in setting position on an active playing WebAudio node through 'pos(position, id)

First pull request ever. I hope I'm doing this right!

It's much more reliable to pause the currently playing WebAudio, then set the position, and resume playback so that the two activities don't interfere with each other.

I found that setting the position of a currently playing WebAudio gave unpredictable results and had to be called a few times before it set to where it needed to go.

The pause should be imperceivable.

Thanks!