WIP: Remove the presented at prefix.

#6670 
Before 
![selection_130](https://cloud.githubusercontent.com/assets/1222368/4753355/667291ee-5ab6-11e4-932c-a3b94aa00cfc.png)

After
![selection_131](https://cloud.githubusercontent.com/assets/1222368/4753369/7e634744-5ab6-11e4-9eaf-727e9e051091.png)


@RoySegall 
I think it's better to try an wrap this with ``<group prefix=", ">`` to avoid the prefix in the ``<text>`` itself

@rbran100 Since we are dealing with standard CSL files, maybe it's worth providing it as a PR, or is this Harvard specific?