Prepare a 4.0.0.0.pre.1 release

This moves the namespace for the entire gem to DPLA::MAP and preps a release.

Before release, ActiveTriples should merge feature/validations-2 so we can pin dpla_map.gemspec to an appropriate version.

The MAP 3.x branch should also be cleaned up to use the DPLA::MAP namespace and the DPLA::MAP::VERSION to manage release number (assuming we think we will ever use that branch).