Dev: Added different Flickr-requestTypes

Added the possibility to use different requestTypes:
- user (default) for requesting your latest images
- set to get the latest photos of a certain set
- gallery to get the latest photos of a certain gallery
- favorites to get your latest favorites

If requesting photos of a set, the ID of the set must be given
If requesting photos of a gallery the URL of the set must be given (there is no way to get the ID of the gallery directly on flickr.com)

If no requestType is given, the requestType 'user' is used to obtain backward-compatibility.