Adding a configurable "unique_id" to DofObject.  Currently works for Ser...


![2dproblem](https://f.cloud.github.com/assets/3036960/1240821/8d20ff54-2a12-11e3-89ba-a804aa920aca.png)
...ialMesh.  
![figure_1](https://f.cloud.github.com/assets/3036960/1248514/0f340c0e-2acf-11e3-8e1f-00fea143e658.png)

There is still more work to do before this will work with ParallelMesh, but I just wanted to post this now to get any "architecture" comments out of the way since this touches some pretty important pieces of the library.