Add Yt::ContentOwner to README

Also fix the broken links to rubydoc.info
Automatically refresh the access token if invalid

An OAuth2 access token from YouTube has a duration of one hour.
After this time, the requests made with the access token return
401 Unauthorized. However, if the refresh token is also available,
then the access token gets refreshed and new requests can be made.
Merge branch 'automatically-refresh-the-access-token'