Gets descMetadata assignment working for all shared fields.
Working assignment of resource_type to descMetadata.
Adds convenience methods for retrieving date of upload and date of modification; adds no paired setters for either of these because everything seems to get set correctly.
NuCoreFile now implements methods defined by metadata_assignment as mass-assignable.  Laying groundwork for eventual rewrite of form code.
Merge pull request #76 from nu-lts/feature/integrate_descmetadata

Feature/integrate descmetadata