Modify DataSet Citations

* Created DataSetCitations cross-reference table, to enable many to
many relationships between DataSets and Citations.  Added
RelationshipTypeCV to this cross-reference table so that Citations can
either be the SourceOfDataSet or CitationForDataSet. Deleted old
foreign key and created new ones.
* Changed field names in Citations to be equal to those core, required
fields recommended by DataCite.org.  See Table 1 in
http://dlib.org/dlib/january11/starr/01starr.html
* Added CitationExtensionProperties cross-reference table, to allow the
use of optional citations fields, such as those suggested by
DataCite.org or those in the RIS convention
(http://en.wikipedia.org/wiki/RIS_(file_format)).  That will be the
recommended best practice.