New TileManager

This removes all tile queueing/loading specific code from Layer.Grid
and creates a new class that manages tile loading and caching.

tests/list-tests.html

@@ -228,6 +228,7 @@
     <li>Tile/Image.html</li>
     <li>Tile/Image/IFrame.html</li>
     <li>Tile/UTFGrid.html</li>
+    <li>TileManager.html</li>
     <li>Tween.html</li>
     <li>Kinetic.html</li>
     <li>Util.html</li>