Flickr-photos-api

* Retry "Server disconnected without sending a response" errors from the Flickr API.

* Retry "connection reset by peer" errors from the Flickr API.

* Remove irrelevant information from the traceback shown when the library throws a `UnrecognisedFlickrApiException`.
[64](https://github.com/Flickr-Foundation/flickr-photos-api/issues/64)

* Add a new method `get_photo_contexts(photo_id)` which returns information about all the contexts where this photo appears -- albums, galleries, and groups.
* Improve the way error codes from the Flickr API are handled. If you're calling the library, this means you'll get more human-readable error messages. If you're working on the library, you have more flexibility in how error codes are mapped to Python exceptions.
* Fix a bug where getting photos from the photostream of a user who didn't have a `realname` set would throw a `KeyError`.

This adds a new method `list_commons_institutions()` which returns a list of all the institutions in [the Flickr Commons](https://commons.flickr.org).

Get rid of the `SinglePhotoWithSizes` type and roll back to the `SinglePhoto` type, which maintains better back-compatibility.