move playground icon to SVG

Implements part of #1165 by changing the icon for `amenity=playground` to SVG (as requested by @mkoniecz in #1012). 

Symbol ([playground-16.svg](https://github.com/nebulon42/osmic/blob/master/amenity/playground-16.svg))
![playground-16](https://cloud.githubusercontent.com/assets/3531092/5554146/5197547a-8c44-11e4-9e41-6269e77754e0.png)

http://www.openstreetmap.org/#map=19/46.79778/13.49265
before:
![playground_before](https://cloud.githubusercontent.com/assets/3531092/5554148/869d1c2c-8c44-11e4-81ff-0753a15e2ae8.png)

after:
![playground_after](https://cloud.githubusercontent.com/assets/3531092/5554149/8c8183da-8c44-11e4-9e0b-4dba2a61977e.png)

The same symbol is also used by the HOT style.