Move :iframe_attributes to .embed_code method, add :iframe_attributes

I believe it makes more sense to have the #embed_code as a method on the Provider class.

This allows us to cache the video info from the provider's API, and then generate embed code with different iframe attributes as needed.

I have also added the possibility to specify additional :url_attributes via hash (for example :autoplay => 1) etc.

Would this make sense to be pulled in?
(I bumped the version to 1.3.3.)

Best, Tomas