Finish gemification

This removes unused MIME Type controlled vocab, moves gemspec from
dpla_map to dpla-map, and includes RDF::Vocabularies individually so
they get added when loaded as a gem.
Move shared factories into lib

This allows `require 'dpla/map/factories'` to use this gem's factories
in another gem.
Merge pull request #4 from dpla/feature/gem-factories

Finish gemification; create shared factories