move museum icon to SVG

Implements part of #1165 and moves `amenity=museum` to SVG.
Icon can be found here: https://github.com/nebulon42/osm-icons/blob/master/amenity/museum-16.svg

http://www.openstreetmap.org/#map=19/46.79838/13.49323
before:
![museum_before](https://cloud.githubusercontent.com/assets/3531092/5531186/143ba652-8a29-11e4-97c0-3fad5b5a98b5.png)

after:
![museum_after](https://cloud.githubusercontent.com/assets/3531092/5531191/1ac136f4-8a29-11e4-9756-9838fbcef0a3.png)
our existing naming convention would be museum.16.svg - have we decided to change that?