Add/fix Jacobian computations for interior facets

I've cherry-picked this from the extruded facet branch.  This commit has nothing to do with extruded meshes, and simply adds separate Jacobian calculations for interior facet integrals (which are required since the coordinates for the two cells are interleaved).  The initial three-line edit is an unrelated bugfix.

Obviously this code needs corresponding FFC changes to actually use the new functions; that's on the way, but this can be merged immediately.