refactor TileLayer:

- set specific TileRenderer in sub-class constructor
- remove TileLoader generics from TileLayer

vtm-android-example/src/org/oscim/android/test/PathOverlayActivity.java

@@ -36,7 +36,7 @@ public class PathOverlayActivity extends BitmapTileMapActivity {
 	@Override
 	public void onCreate(Bundle savedInstanceState) {
 		super.onCreate(savedInstanceState);
-		mBitmapLayer.getTileRenderer().setBitmapAlpha(0.5f);
+		mBitmapLayer.tileRenderer().setBitmapAlpha(0.5f);
 
 		createLayers(1, true);