Fix constants export (#121)

Author: dnischeta

src/index.js

@@ -14,7 +14,6 @@ export {default as DirectionalLight} from './lights/DirectionalLight';
 export {default as Frustum} from './math/Frustum';
 export {default as Geometry} from './Geometry';
 export {default as Mesh} from './Mesh';
-export {default as libConstants} from './libConstants';
 export {default as Line3} from './math/Line3';
 export {default as MultiSprite} from './MultiSprite';
 export {default as MultiSpriteMaterial} from './materials/MultiSpriteMaterial';
@@ -42,3 +41,6 @@ export {default as quat} from './math/quat';
 
 import * as Math from './math/Math';
 export {Math};
+
+import * as libConstants from './libConstants';
+export {libConstants};