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.
Merge pull request #109 from rnorris/master

Add support for natural POIs: Trees and Peaks.