Support feeds with stories with scheme-less urls

Example is https://blog.golang.org/feed.atom.
Urls in this feed are referenced to as scheme-less urls
(//blog.golang.org/context).

This commit adds normalization of story urls using the feed url. If a
story url doesn't have a scheme it will use the scheme of feed's url.
Refactor url normalization in StoryRepository
Refactor StoryRepository#normalize_url for improved readability
Merge pull request #329 from pascalw/bugfix-schemeless-story-urls

Support feeds with stories with scheme-less urls