implement specular, URDF non-standard specular part (see sphere2.urdf) and SDF specular support.

pybullet.changeVisualShape(obUid,linkIndex,specularColor=[R,G,B]) and Bullet C-API b3UpdateVisualShapeSpecularColor
Bug fixes in b3ResourcePath::findResourcePath resolution.
add stadium.sdf and roboschool/models_outdoor/stadium assets https://github.com/openai/roboschool/tree/master/roboschool/models_outdoor/stadium
minor fixes to obj2sdf

data/roboschool/models_outdoor/stadium/stadium.mtl

@@ -0,0 +1,16 @@
+newmtl stadium_white
+Ns 96.078431
+Ka 0.000000 0.000000 0.000000
+Kd 1.000000 1.000000 1.000000
+Ks 0.500000 0.500000 0.500000
+
+newmtl stadium_grass
+Ka 0.000000 0.000000 0.000000
+Kd 0.000000 0.500000 0.000000
+Ks 0.000000 0.000000 0.000000
+map_Kd stadium_grass.jpg
+
+newmtl stadium_dirt
+Ka 0.000000 0.000000 0.000000
+Kd 0.600000 0.400000 0.000000
+Ks 0.000000 0.000000 0.000000