adding “usage” endpoint
adding “creation” endpoint
Merge pull request #39 from jfairley/date-range-endpoints

Adding support for "usage" and "creation" search endpoints
Add/update/delete permission targets.
Preserve the principals hash instead of splitting it into users and groups.
Tests for permission targets.
Convenience methods to get and set users and groups.
Re-create the principals hash from users and groups.
Use friendly names for permissions, e.g. admin, read, deploy, delete, annotate.
Tests for getters and setters.
Clean up from_hash method.  Replace overly-similar method names.
Cleanup tests for easier debugging.
Manage principals and permissions in a Principal class.
Sort arrays of abbreviated permissions.
Unit tests for the Principal object.