Consider if the /photos/list.json and /photo/:id/view.json APIs should return a sharing token

[jmathai]

Sharing tokens are becoming a more heavily requested feature (for example, photo/openphoto-wordpress#20).

The easiest way to distribute sharing tokens is to include them with photo objects in the API.

The biggest question is how to handle multiple sharing tokens (do we generate a new one every time?) and how to handle expirations.

Essentially, if the sharing tokens are going to be used regularly then they need to be present with the photo object else it's too much work to make subsequent calls.