Issue 17: As a faculty I want to upload my dataset and describe it.

Adds test coverage for GenericWork
Adds rspec matchers for single-value and multivalue fields
Adds example set for has_dc_metadata (used when testing GenericWork & Dataset)

Adds Dataset model as a type of Work & CurationConcerns::DatasetsController as subclass of CurationConcerns::GenericWorksController 
Adds DatasetActor & wires Datasets into CurationConcerns as appropriate
Adds Feature test to ensure full flow of creating & editing Datasets works

Story Ticket: https://github.com/ndlib/planning/issues/17

Note: I didn't do anything with the sample dataset file that @rickjohnson provided (34MB).  It wasn't clear what I would need to do with it in the test suite that would warrant adding a 34MB file to the git repository.