This commit is part of the webmentions support. These tags help the website
receiving the webmention parsing the mention. Only a minimal amount of
informations have been tagged: I did not wanted to disrupt a lot of views.

We still could extend the h-entry by including the stories comment in them. It
would however spread this whole tagging across several views instead of being
contained in a single one.

We add a cron triggered script which send webmention for each new story submitted.

Sponge.fetch will now return the http headers as well as the response
body. The existing codebase has been refactored according to this
change.