Add support for result/variant sets

This includes models, resources, and pages for interacting with result sets. Any additional changes or features can be requested in new issues and revisions can be made against master.
As of openpyxl 2.0.0+, they changed the property for accessing the method to just `value`. See what I did in Origins to accomodate: https://github.com/cbmi/origins/commit/4aa1cd7ac4181458a58e89fd05f0e72ed208e2e0