Use FileFinderUser role to look for files that should be scanned

This approach was suggested in pull request #10. It adds a 'finder'
attribute to [LocaleTextDomain] which allows to specify so-called finders
to search for files, and thus basically implements the enhancement described
in issue #1.
Add test data and tests for FileFinderUser-based file list retrieval
Document the new 'finder' attribute of the LocaleTextDomain plugin
Merge pull request #11 from mstock/feature/scan-additional-files

Use FileFinderUser role to look for files that should be scanned