Make VBO messages for removing terms from nodes to be more detailed.

#1531 

This PR addresses https://github.com/openscholar/openscholar/issues/1531#issuecomment-46575287.

It makes the messages for VBO for removing terms more detailed. For example, having 3 classes, each with the following terms:
![selection_259](https://cloud.githubusercontent.com/assets/4497748/3350748/4c3d32f2-f9d1-11e3-981a-add0e195c24f.png)
![selection_260](https://cloud.githubusercontent.com/assets/4497748/3350749/4c464c2a-f9d1-11e3-8938-443aa13b76cb.png)
![selection_261](https://cloud.githubusercontent.com/assets/4497748/3350750/4c466624-f9d1-11e3-8555-7d1fbf720880.png)

When choosing to remove the terms: "bar" and "natural science" (both belong to "Class 3", one of them to "Class 2" and none of them to "Class 1") we get this messages:
![selection_262](https://cloud.githubusercontent.com/assets/4497748/3350752/cdfb24ca-f9d1-11e3-922c-31dccabe5ec8.png)