Fix Drawer event emission

Drawer tries to emit events on the parent Viewer object but doesn't have
a reference to it.  Add a reference when constructing the Drawer.

src/drawer.js

@@ -94,6 +94,7 @@ $.Drawer = function( options ) {
     $.extend( true, this, {
 
         //internal state properties
+        viewer:         null,
         downloading:    0,
         tilesMatrix:    {},
         tilesLoaded:    [],

src/viewer.js

@@ -1047,6 +1047,7 @@ function openTileSource( viewer, source ) {
     _this.source.overlays = _this.source.overlays || [];
 
     _this.drawer = new $.Drawer({
+        viewer:             _this,
         source:             _this.source, 
         viewport:           _this.viewport, 
         element:            _this.canvas,