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)


This is a bug, I'm fixing it now. It should use edge_boundary_ids (el, edge_id);