[importer] Implement downloader for HTTP and FTP URLs
[importer] Fix silly typos in Importer::Downloader
Changes the order of the warnings: 1. Reached limit. 2. Upgrade account.  3. 2.1. Welcome message. Fixes Vizzuality/cartodb-management#2694
[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
added option for cdn_url
Merge branch 'develop' into release/mig21