Specify the storage backend class to be used for attachments separately from the default backend. This allows us to treat attachments and other static assets differently
Add the path to DRF 0.4 to the Python path in the WSGI module
Allow developers to store private data on submitted things

- Set the user on the request when authenticating with API key.
- Only allow users logged in by some method other than API key to view private
  data.
- Set show_private_data on a view if the request has show_private in the
  querystring, and the user is allowed to see private data for the resource.
- Have the ModelResourceWithDataBlob mixin detect when a view has
  show_private_data set.
- Update the CacheMixin to not cache 403 responses, so that we can keep taking
  advantage of caching, but still return different status codes for different
  users.