Update cms-primary-datasets.xml

Adding     
256$a Dataset
Closing #91 and #154 
... however, when I look at the [MARC standard for field 256](http://www.loc.gov/marc/bibliographic/bd256.html), I see it says `NR` (=non repetitive).  So it should be treated like `100` or `245`... hence we should make it only one field instance.  So we shouldn't really use the repetitive-256 technique it seems...