JxW in Assembly class has to come from the surface (closes #3345)

When integrating over the face surface, transformed jacobian weights
have to come from the surface.  It was incorrectly implemented and JxW
came from the volume. This also fixes the memory leak introduced by
copying a JxW array into another MooseArray.
@friedmud _fe_neighbor always holds "face" data. That was the original design when we added DG. That's why I did not use the `_face` suffix. It got a little confusing with `_current_neighbor` which is actually the neighbor element.  In this patch I am adding `_current_neighbor_side_elem`, which is the side of the neighbor element. I am not aware that any code would need the volumetric FE. We create one locally in `reinitNeighborAtReference` to compute the volume of the neighbor element. But I do agree that `_fe_neighbor` seems like the volumetric version. So, I'd agree with @permcody to merge this and then fix the names, because it will need slightly more work, I think.