chainable .on/.off and Layer.(before/after)Draw. closes #279

src/Layer.js

@@ -96,6 +96,7 @@
          */
         beforeDraw: function(func) {
             this.beforeDrawFunc = func;
+            return this;
         },
         /**
          * set after draw handler
@@ -105,6 +106,7 @@
          */
         afterDraw: function(func) {
             this.afterDrawFunc = func;
+            return this;
         },
         /**
          * get layer canvas

src/Node.js

@@ -74,6 +74,7 @@
                     handler: handler
                 });
             }
+            return this;
         },
         /**
          * remove event bindings from the node. Pass in a string of
@@ -112,6 +113,7 @@
                     delete this.eventListeners[baseEvent];
                 }
             }
+            return this;
         },
         /**
          * remove child from container, but don't destroy it