Add support for rendering "natural=tree-row".

Very handy for walking navigation, and it prevents from tagging for the
renderer by mapping things as wood/forest instead for a small section between
fields or f.e. near railways.

There are two example renderings at:

 - http://derickrethans.nl/files/dump/tree_row.png
 - http://derickrethans.nl/files/dump/tree_row2.png