Add Flexible media embeds utility

As in https://github.com/twbs/bootstrap/issues/9412 this was plan to add for 3.1 but with release date come closer i am scare that it was delayed for future release so decide to make a pr myself. This is really useful to embed youtube, vimeo videos, 

A quick example of what it does : http://jsbin.com/ogULUma/3

Really look forward for this to make it into BS core.