Add optional support for object set resources and endpoints

This uses a settings-based approach to defining which object set models
to define resources and endpoints for. In addition to the functionality
provided by django-objectset directly, resources support creating sets
based on a DataContext literal, an existing instance or the user's
current session.

Requires django-objectset 0.2.3+
Merge pull request #139 from cbmi/issue-136

Add optional support for object set resources and endpoints