Fixed stereo input and comparison

* Finally figured out how to get stereo input. Turned out the master constraint echoCancellation=false was all that was needed.
* testInputVolume() now returns the calculated db or volume for the provided buffer and channel.
* Rounded the db value as we do not more accuracy than a float without decimals.
>Mono microphone implies same audio DB on both channels.. but the opposite is not true (you could just be incredibly lucky and get the same DB, no?)
Yes it could be the same if rounding without any decimals is applied (as it is now) but having higher resolution should make it possible to determine if the channels are from different microphones. The chance of 8192 samples being the same from different microphones is unlikely.
I guess if I just do the rounding on the db values displayed to the user and use the raw db result (which it was originally) in the comparison, i.e.  it should offer enough resolution to determine that the inputs are from two different microphones.