added option for calculating zooms, `less_bounce`, removed setting `L.Icon.Default.imagePath`

`less_bounce` Less map bounce when calculating zoom, false is good when there are clusters of tightly grouped markers.

Setting `L.Icon.Default.imagePath` was leftover from when I was using bitmapped images for markers. It's no longer needed and may be causing issues with the authoring tool. reference to issue #91 @jywsn let's see if that fixes the authoring tool issue.