allow pybullet_envs.deep_mimic.testrl --arg_file run_humanoid3d_backflip_args.txt to perform a backflip. Can only backflip twice, then drops on ground.

this deepmimic is still very slow, due to slow mass matrix/inverse dynamics computation. once spherical motor drive is enabled, it should be fast(er)
move pd_controller_stable to pybullet_utils for easier re-use
add plane_transparent.urdf to pybullet_data
allow spacebar in keyboardEvents (Windows for now)

examples/CommonInterfaces/CommonCallbacks.h

@@ -11,6 +11,7 @@ typedef void (*b3RenderCallback)();
 enum
 {
 	B3G_ESCAPE = 27,
+	B3G_SPACE = 32,
 	B3G_F1 = 0xff00,
 	B3G_F2,
 	B3G_F3,
@@ -40,7 +41,8 @@ enum
 	B3G_SHIFT,
 	B3G_CONTROL,
 	B3G_ALT,
-	B3G_RETURN
+	B3G_RETURN,
+	
 };
 
 #endif