Add support for natural POIs: Trees and Peaks.

Since there are currently +3M trees and +1M peaks in the OSM DB,
it's about time these are supported as nice icons and directly create-able in Potlatch2.

Icons have been derived from the Maki icon set (https://github.com/mapbox/maki)
natural_tree.n.24.png is copied from park2-24.png
natural_peak.n.24.png is from triangle-24.png
 (with the colour changed to that of the peak symbol in the standard mapnik rendering)
The n.16 versions are simply scaled down versions of the n.24 images.