Primitives: fixed cleanup by setting "alive" flag after Body creation

- I was getting strange behaviour and crashes on exit, because bodies
  were still being referenced by the World. The "alive" flag for Circle, Rect
  and Polygon was never set, so destroy() wasn't getting called in the
  destructor.

src/ofxBox2dCircle.cpp

@@ -41,6 +41,8 @@ void ofxBox2dCircle::setup(b2World * b2dworld, float x, float y, float radius) {
 
 	body  = b2dworld->CreateBody(&bodyDef);
 	body->CreateFixture(&fixture);
+
+    alive = true; // Need this, so the Body gets cleaned up in destructor
 }
 void ofxBox2dCircle::setup(b2World * b2dworld, ofVec2f &pts, float radius) {
     setup(b2dworld, pts.x, pts.y, radius);

src/ofxBox2dPolygon.cpp

@@ -235,6 +235,8 @@ void ofxBox2dPolygon::create(b2World * b2dworld) {
 
 	// update the area and centroid
 	updateShape();
+
+    alive = true; // Need this, so the Body gets cleaned up in destructor
 }
 
 //------------------------------------------------

src/ofxBox2dRect.cpp

@@ -59,7 +59,8 @@ void ofxBox2dRect::setup(b2World * b2dworld, float x, float y, float w, float h)
 
     // update the rectShape
     getRectangleShape();
-
+
+    alive = true; // Need this, so the Body gets cleaned up in destructor
 }
 
 //------------------------------------------------