Prison area

Isolating undisputed style from combined proposal #771
This provides rendering for areas of amenity=prison, and closes

https://trac.openstreetmap.org/ticket/2512

Prison was apparently implemented in xml and did not make it to carto. 

The design adopts the hatching style we have for military areas/barracks, just in a neutral grey and vertical, associating bars, as in the original implementation. Hatching being used, as @mkoniecz pointed out, for rather inaccessible areas. Or, in this case, difficult to leave ;-)

Icons is powers of two and allows global alignment.

![pr](https://cloud.githubusercontent.com/assets/8266355/3714300/78cc44d0-15a0-11e4-87d1-41389387fb5f.png)