Moved vertices for Box fixture back on the stack

There is no reason why these temporarily calculated vertices should be a
member variable. It's faster to have it on the stack.
Improved Box2DBox::createShape

Use b2PolygonShape::SetAsBox, which should be a little more efficient
than setting the box as a polygon since it avoids a bunch of checks and
calculations.

Also, an initial rotation property set on a Box fixture is now
supported, though changes to it are not yet applied.