Finish gemification; create shared factories

Completes gamification of `dpla-map` for inclusion in other projects.

  - Removes the RDF::MIME vocabulary which was unused (and won't be used in MAPv4) and causing problems with ruby's Mime library.
  - Moves gemspec from `dpla_map.gemspec` to `dpla-map.gemspec` to reflect change in gem name.
  - Includes custom RDF::Vocabularies explicitly instead of as a relative glob, so they load properly when the gem is used.
  - Moves factories into `lib/dpla/map` so they can be included in a project that uses `factory_girl` by adding `require 'dpla/map/factories` to the project's `spec_helper`.