allow to use colors from MJCF file as option (default to random Google colors), use p.loadMJCF(filename, flags=p.URDF_MJCF_COLORS_FROM_FILE

fix quadruped.py example.
add PyBullet.isConnected() API, more friendly than PyBullet.getConnectionInfo()["connected"]

data/plane_transparent.urdf

@@ -15,7 +15,7 @@
 				<mesh filename="plane.obj" scale="1 1 1"/>
       </geometry>
        <material name="white">
-        <color rgba="1 1 1 .9"/>
+        <color rgba="1 1 1 .7"/>
       </material>
     </visual>
     <collision>