Added support for Flickr collections
- Shows the latest photos from collection
- Store Flickr API key in gallery settings
- No more field validation in flickr tab (will be handled at gallery level)
- Support the PTG "batch_size" setting; zero means no limit
Merge pull request #1 from savoirfairelinux/collections2

Added support for Flickr collections
Fixed tests module
Give every i18n string an identifier and a default value
More error logging for users, sets and collections
Make code a little more bulletproof, and mention username/ID in error messages.
Added documentation
Mention in the doc that many different api keys are supported
Merge pull request #2 from savoirfairelinux/collections2

Tests, logs, i18n strings