Manage oEmbed responsive

Bootstrap manage the output of responsive embeds:
http://getbootstrap.com/components/#responsive-embed

This simple function wrap the oEmbed HTML with a div with `class="embed-responsive embed-responsive-16by9"` in order to make it responsive.

I know, the size of all oEmbed will be 16/9, I believe is now a big deal because oEmbed shortcode are usually used for videos, and right 16/9 is a kind of standard.

Cheers!