Add docstring and make contact! work with the screen map

src/play_clj/core_listeners.clj

@@ -34,6 +34,7 @@
       false)))
 
 (defn ^:private gesture-listener
+  "Internal use only"
   [{:keys [on-fling on-long-press on-pan on-pan-stop on-pinch on-tap on-zoom]}
    execute-fn!]
   (reify GestureDetector$GestureListener

src/play_clj/g2d_physics.clj

@@ -216,8 +216,8 @@
 
 (defmacro contact!
   "Calls a single method on a [Contact](http://libgdx.badlogicgames.com/nightlies/docs/api/com/badlogic/gdx/physics/box2d/Contact.html)"
-  [object k & options]
-  `(u/call! ^Contact ~object ~k ~@options))
+  [screen k & options]
+  `(u/call! ^Contact (u/get-obj ~screen :contact) ~k ~@options))
 
 (defn find-body
   "Returns the first entity in `entities` whose body matches `body`"