Handle small roundabouts, fixes #516.

This gives us ~51k additional roundabout instructions.
Guidance handes sizes internally.

Note: it is highway=mini_roundabout but junction=roundabout

References:
- http://wiki.openstreetmap.org/wiki/Tag:highway%3Dmini_roundabout
- http://wiki.openstreetmap.org/wiki/Tag:junction=roundabout
- http://taginfo.openstreetmap.org/tags/highway=mini_roundabout

profiles/foot.lua

@@ -162,7 +162,7 @@ function way_function (way, result)
   end
 
     -- roundabouts
-  if "roundabout" == junction then
+  if "roundabout" == junction or "mini_roundabout" == highway then
     result.roundabout = true
   end