More garbage collection friendly (on repeated calls) Geometry.computeBoundingBox() and Geometry.computeBoundingSphere()

fixed #2728, Moved geometry primitives out of src/core into src/math.  src/core is now exclusive scene primitives.

add equals, clone to Box2, Box3, Plane and Sphere. Adopt +Inf/-Inf in Box2 and Box3 per @mrdoob'd recommendation.  Begin Box3 unit test.

move away from static fromXXX-style constructors to setXXX style member functions widely used in ThreeJS.

Plane,Box3,Sphere improvements: static constructors, code simplficiation, optimizations.  Box3 made more robust via true empty (+MAX_VALUE,-MIN_VALUE).