Adding in support for undocumented count_faves

Hi Toby,

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/

There are some other undocumented things mentioned in that discussion as well that might prove interesting.  But for scratching my particular itch, I was happy with count_faves.

Kevin.