Use Float32Array#subarray instead of Array.slice

...Also use a loop instead of Math.max

slice vs subarray: http://jsperf.com/float32array-array-slice-call-vs-subarray
for loop vs Math.max: http://jsperf.com/find-max-in-float32array
getPeaks (new vs old): http://jsperf.com/wavesurfer-getpeaks

This guy was running something like 450ms on a 3:30 audio file for me, so I wanted to see if I could get that down a bit. Same file with the new function runs in about 40ms now.

Awesome work on this library. Huge fan.