minor code cleanup
Add support for count_faves

For some hobby code I'm writing I needed to know the number of favourites a given photo has when
calling getPublicPhotos() without querying each photo separately. Ran into a dead end with the
Flickr documentation and then by chance stumbled across this Flickr discussion where the
undocumented count_faves was mentioned:

https://www.flickr.com/groups/api/discuss/72157636063789543/
Merge branch 'kevinspencer-countfaves'