Don’t raise error if subscribing to your channel

YouTube does not let people subscribe to their own channels and, even worse,
raises a 500 error in this case, rather than a 4xx.

Instead of bubbling this error up, this commit changes the behavior of
`channel.subscribe` to simply return nil in the case described above.

`channel.subscribe!`, on the other hand, will still raise the error.
Autoload more models when requiring `yt`

Apart from Account and ChannelOwner, it also makes sense to make
other models available on `require yt`, so they can be initialized such as:

    Yt::Channel.new ...
    Yt::Playlist.new ...
    Yt::PlaylistItem.new ...
    Yt::Video.new ...
Merge branch 'don-t-try-to-subscribe-to-your-own-channel'