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.
If you have a random variable, the more samples you average, the more information you lose if they are the same variable or not... imagine you roll a dice 1000 times.. on average you will get 3.5, which could be very much likely to the average of throwing the dice again 1000 times... on the other hand comparing 1000 dice rolls to see if they are the same dice roll sampling is a very different problem.

I guess if you keep it double it will give you enough precision. But is it that much harder to make it exact? You could simply calculate how many samples yield different values.