Add the new vendor gem 'rest-client' to retrieve image's headers
Fix the 'awesome_print' and the 'letters' dev deps in the runtime scripts
Add the new option `--fill` to fill cards with examples or media files
Add an URL existence check to find outdated images in the cards
Fix a bunch of minor issues
Version bump to 0.0.4
Pre release actions: new gemspec + version bumped
Merge branch 'release/0.0.4'