Added Citations and SpecimenTaxonomicClassifiers

* Added Citations table and foreign keys to Datasets,
ReferenceMaterialValues and SpecimenTaxonomicClassifiers
* Added SpecimenTaxonomicClassifiers cross-ref table and foreign keys
to parent tables.
Added CitationExternalIdentifiers

* Added CitationExternalIdentifiers cross-ref table in
ExternalIdentifiers schema, and appropriate foreign keys and comments
* Added comments to fields in TaxonomicClassifierExternalIdentifiers
cross-ref table
Removed Datasets from ExternalIdentifiers

Removed the Datasets entity and its bridge table from the
ExternalIdentifiers schema. Dataset linkage to an ExternalIdentifier
will be through its citation.
Merge pull request #32 from UCHIC/SourceInfo_FeatureBranch

Merging SourceInfo feature branch into Master.