Added support for Flickr API authentication and also added FLICKR_SETS_INCLUDE

The standard flickrapi code will only pull down 10 photo sets and the photos included in those sets seem restricted. I have added code to handle standard Flickr API authentication. All you have to do is include the API Secret in the settings. You get a one time authentication web page. You are then prompted to press ENTER to continue.

I have also added a FLICKR_SETS_INCLUDE setting. This take precedence over the EXCLUDE counterpart and allows the user to choose the photo sets they want to include.