Moved VersionCode to DataSets

It seems that versioning is really only important when a dataset gets
“published” or made public, whereas revisions to Results are likely not
worth tracking.  Therefore, I:
* Deleted ResultVersions table
* Added VersionCode and VersionOfDataSetID to DataSets table.
Is a VersionExplanation necessary for a Dataset?  Can that function be
fulfilled by the DataSetAbstract?