Prepare for Bullet 2.86 release, update version to 2.86 (release will be tagged soon)

protect some C-API methods against negative dof indices
add triangle mesh import from MJCF files
update to recent pybullet quickstart guide pdf

src/LinearMath/btScalar.h

@@ -29,7 +29,7 @@ subject to the following restrictions:
 #include <float.h>
 
 /* SVN $Revision$ on $Date$ from http://bullet.googlecode.com*/
-#define BT_BULLET_VERSION 285
+#define BT_BULLET_VERSION 286
 
 inline int	btGetVersion()
 {

src/LinearMath/btSerializer.h

@@ -505,7 +505,7 @@ public:
 
 			buffer[9] = '2';
 			buffer[10] = '8';
-			buffer[11] = '5';
+			buffer[11] = '6';
 
 		}