Entity creation

[jimkang]

This is an initial PR for the purposes of discussing the changes so far to support the creation of freestanding entities that aren't necessarily associated with documents.

Additions:

• route that maps the /api/freestanding_entities endpoint to FreestandingEntityViewSet.
• FreestandingEntityViewSet to handle GET and POSTs to the endpoint.
• Unit test of existing _get_or_create_entities method.
• Test for FreestandingEntityViewSet. Currently, this only runs when only the documentcloud/documents tests are run. When all of the tests are run with inv test this test fails.
• EntityAdmin so that freestanding entities can be viewed in the Django admin UI.

[jimkang]

@mitchelljkotler Just added the latest requested changes.