Add symbol black_horse and green_horse (horse heads)

[Hufkratzer]

Foreground symbols black_horse and green_horse are supported by wanderreitkarte http://www.wanderreitkarte.de/symbols_de.html

black_horse on white square:
http://riding.waymarkedtrails.org/de/relation/3221092
http://www.wanderreitkarte.de/index.php?lon=7.1081&lat=51.0122&zoom=14
black_horse on black_circle:
http://riding.waymarkedtrails.org/de/relation/3985399
http://www.wanderreitkarte.de/index.php?lon=13.7910&lat=51.2592&zoom=14

Here how the horse heads in Saxony look like:
http://www.forsten.sachsen.de/wald/241.htm
http://freizeit-wanderreiten.de/?page_id=31

green_horse I could not find in any osmc:symbol, but there are routes that have it in real life, in the symbol description in OSM, and in the wanderreitkarte on a green_frame background:
http://www.hartenfels-84.de/Reiten/reitwege.htm (click photos)
http://www.wanderreitkarte.de/index.php?lon=13.0246&lat=51.4548&zoom=14
http://wiki.openstreetmap.org/wiki/Dahlener_Heide/Reitwege

[Hufkratzer]

In https://commons.wikimedia.org/wiki/File:1_Cav_Shoulder_Insignia.svg
is a nice horse head in svg format that I think you could use.

In a background circle it would look something like this:

(like in http://www.wanderreitkarte.de/index.php?lon=13.4228&lat=50.6823&zoom=16)

P.S.
white_horse would also be useful, especially for Austria,
see http://www.nordweide.at/de/reitwegenetz.html