Add missing tests for playlists’ snippets
Allow `has_one :status` on any resource

This commit allows any resource to be initialized without a status,
and successively request the status to the YouTube API.
Move account’s authentication methods into Module

In this way, we don’t abuse the `has_one` metaphor and we can
reduce the duplication of code among the real `has_one` associations.
Remove duplication in `has_one` associations

Seven `has_one` associations were defined in the same way, so the
code can be simplified by declaring `has_one` programmatically.

This commit does not change any functionality, with two exceptions:
"account.channel" and "video.rating" were the only two `has_one`
associations that would return +nil+ when the association was not
found, rather than raising an Error. Now they raise an error too,
in coherence with the other associations.
Merge branch 'extracting-status'