added basic serialization for several constraints including btPoint2PointConstraint, btHingeConstraint, btSliderConstraint, btConeTwistConstraint, btGeneric6DofConstraint

(no motor support or advanced settings yet)
added btStaticPlaneShape serialization
Added toggle in cmake for BenchmarksDemo to enable/disable graphics rendering

Extras/Serialize/BulletFileLoader/btBulletFile.h

@@ -23,6 +23,7 @@ subject to the following restrictions:
 
 #define BT_COLLISIONOBJECT_CODE MAKE_ID('C','O','B','J')
 #define BT_RIGIDBODY_CODE		MAKE_ID('R','B','D','Y')
+#define BT_CONSTRAINT_CODE		MAKE_ID('C','O','N','S')
 #define BT_BOXSHAPE_CODE		MAKE_ID('B','O','X','S')
 #define BT_SHAPE_CODE			MAKE_ID('S','H','A','P')
 
@@ -44,6 +45,8 @@ namespace bParse {
 
 		btAlignedObjectArray<bStructHandle*>	m_collisionShapes;
 
+		btAlignedObjectArray<bStructHandle*>	m_constraints;
+		
 		btBulletFile();
 
 		btBulletFile(const char* fileName);