Vector2 + Vector3 enhancements

- removed <Vector2>.divideScalar() mrDoob's "inv* pattern" for faster
resolution + simple zero attribution;
- better use of chain, when readable, since minifiers can't spot it;
- more readable var declarations;
- <Vector2>.fromAttribute(): better handling for the offset argument;
- <Vector3>.crossVectors() is faster. Removed bad variables
(http://jsperf.com/three-js-vector3-crossvectors-opt);
- caching matrix elements, used more than twice (js gold rule), that
were not being cached;
- micro changes.