I have found a sollution, maybe other like this too.
To do it yourself you would have to override the WP funtion that handles oEmbeds and then add the new code.
I found this plugin that does that and even more:
http://wordpress.org/plugins/advanced-responsive-video-embedder/
Works on my site and I haven't found any conflicts with the Decode theme.
Resizing the window now also adjusts the height so the dimensions stay the same. You can restrict the max width if you like or set it to fill the post width.
Example: http://www.felixklaar.nl