[importer] Return absolute path in Downloader#candidate
[importer] Downloader returns an empty candidate if no valid URL passed
[importer] Loader#run takes no arguments
[importer] Implement Candidate to store candidate file name, paths, etc.
[importer] Implement Job#filepath
[importer] Make runner use Downloader to download files from urls
[importer] Add acceptance spec for files downloaded from Google Fusion Tables
Implement DataRepository::Filesystem::Local#fullpath_for