Handle minPxPerSec differently to allow for zooming

This PR updates the wavesurfer prototype to assign this.minPxPerSec before using it. This allows you to modify minPxPerSec externally and then drawBuffer, effectively adding a 'zoom' capability. It also adds a 'redraw' event that is triggered when drawBuffer() is called, allowing plugins (timeline, for example) to listen for this event and redraw accordingly.