Adding support for more image sizes

I've added support for some of the missing sizes as described [here](http://www.flickr.com/services/api/misc.urls.html) and [here](http://www.flickr.com/services/api/flickr.photos.getSizes.html). I've also added 2 other sizes ('Large 1600' and 'Large 2048') which aren't officially documented in the API, but are returned by the API calls as well as used on the Flickr website (e.g. [here](http://www.flickr.com/photos/eonns/8654159639/sizes/k)).

In the second commit, I made the Size class implement Comparable to make it easier to compare two Size objects based on the documented lengths of the images' longest side.