Render Putt-putt / miniature golf

This pull-request is supposed to resolve issue #518.

Changes:
* e4d593b ways tagged with leisure=miniature_golf are rendered in the same green color as grown up golf-courses in zoom>=15
* 9c2ec39 ways and nodes tagged with leisure=miniature_golf render with [a new symbol](http://www.sjjb.co.uk/mapicons/png/sport_minature_golf.p.20.png) and their name (if available) in zoom>=17

To better distinguish golf-courses from minigolf-courses, maybe [a golf-icon](http://www.sjjb.co.uk/mapicons/png/sport_golf.p.20.png) should also be added to golf-courses and/or the colors should be coloured with different greens in the future?

At the moment many minigolf-courses [have name="Minigolf"  (~555)](http://taginfo.openstreetmap.org/tags/name=Minigolf) or similar set, but nevertheless there are many situations, where minigolf-courses have a meaningful name. So I decided to show also the name of it.

I know, that this is maybe not the most urgent issue, but its the first change I made to a CartoCSS-style, so I've picked a rather easy task. Hopefully my changes are alright, but I'm open for any comments and suggestions.