Add photo hash as a tag

Adding a hash of the photo as a tag to allow finding duplicate photos in
the future.  Photos that are duplicate will have the same hash and can
then be safely deleted.
Fix API_KEY and SHARED_SECRET

API key and secret should be read from the config file and not from the
Conf module anymore.
Add API key and shared secret to config module

It seems that even though Flickr calls it "shared secret", it is really
meant to be something rather "public".  Several other projects seem to
use it this way (namely Gnome shotwell [1]).  This simplifies the code a
little bit too.

  [1]: https://git.gnome.org/browse/shotwell/diff/plugins/shotwell-publishing/FlickrPublishing.vala?id=ead3a6f75c001059980cc4a2c287feb0622f3c89