add rendering for natural=saddle

This addresses #718. natural=saddle is rendered from z15 onwards as peaks are more important and saddles would clutter the rendering. I'm not sure if z14 would also be appropriate. The rendering is the same as for peaks with another symbol.

An example:
http://www.openstreetmap.org/#map=15/46.9505/13.3299
![openstreetmap-carto_110d6b](https://cloud.githubusercontent.com/assets/3531092/4798428/993d3588-5e13-11e4-850f-594711dd575d.png)

Old example (same location) to which https://github.com/gravitystorm/openstreetmap-carto/pull/1096#issuecomment-60584317 is referring:
![natural_saddle](https://cloud.githubusercontent.com/assets/3531092/4782462/cf5d8246-5cf4-11e4-836c-15356179dfc0.png)