Updated SerialMesh::stitching_helper to enable stitching on node and edge boundary IDs

This allows us to stitch meshes like in the screenshot.

![node_and_edge_stitching](https://cloud.githubusercontent.com/assets/3017801/3892216/8fbe5bbe-2230-11e4-96fc-6cc8981571c6.png)


Good catch.  Feel free to squash all the bug fixes/suggestions down into a single commit and force push the branch.  I know @roystgnr is against losing history like this, but I claim that this extra history is just "noise" in case you ever have to git bisect...