Update Flickr demo project

Completely refactored the Flickr demo project for better separation of Flickr-interfacing code from the rest of the app. OAuth functionality is now inside of FLClient and out of the way of the AppDelegate. So much cleaner!
Change Flickr prefixes from FL to BDBFlickr
Change URL scheme
Remove unnecessary "description" extra on getPhotos call
Remove unnecessary properties in Photo and Photoset models
Cleanup
Forward-declare BDBFlickrPhotoset
Define USE_NSURLSESSION macro
Use ivar directly in init method
Merge pull request #16 from bdbergeron/bdbergeron-flickrDemoRefactor

Refactor Flickr demo project