banners, posters and fanart id

[louman]

Hi, first thanks for the library.

I was using the original tvdb gem but decided to use yours because it seems more complete. I just found one flaw, at least on my opinion.

Calling for ex dexter.posters('en'), the object returned doesnt have poster's tvdb_id. I think would be a good idea return the id so we can keep track easily if we already have saved that image.