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.
Are these meant to hold "face" FE objects?  If so... we should probably name it with `_face` on the end.  That goes for several other associated variables as well.