Presentation 3.0 Alpha Support

[cjcolvar]

This PR adds support for IIIF Presentation 3.0 Alpha (as of 3/23/2018). Given that clients which consume the manifest will take time to support 3.0 manifests, the approach taken in this PR is to create a new namespace (V3) that subclasses and overwrites as necessary the existing classes that target 2.0 instead of updating in place. To generate Presentation 3.0 manifests, use the IIIFManifest::V3::ManifestFactory instead of IIIFManifest::ManifestFactory passing the same presenters.

See http://prezi3.iiif.io/api/presentation/3.0 for more details on how the manifests generated will be different than the default V2 manifests.

Other changes in the PR:

• Explicitly wrapping IIIFCollection in the IIIFManifest module

TODO:

• Verify output by running 2.0 manifest through converter (https://github.com/IIIF/prezi-2-to-3) and compare with generated 3.0 manifest
• Add format attribute to choice items

[cjcolvar]

I'd like to get this reviewed and maybe merged prior to the TODO listed above. If the test reveals any issues then new PRs could be created to fix them.

[tpendragon]

This all looks fine to me. It's ridiculously complex, but that's largely because of the current patterns in place. I'm excited to see the gem being able to generate V3 manifests in addition to V2!