Simple but yet functional, I like plugins like this.
But I have to say this surely needs to have max-width set else it is not responsive.
Also the statement on the wiki that he would fall back to 100% if
$content_width is not set is not true. It actually falls back to fixed 300px on top of error messages popping up witch kind of sucks.
I don't like the general idea to use
$content_width since many themes do not include it. It should just fall back use no width at all by default witch equals to
auto in CSS and that is the BS default and just fine IMO.
I maybe open up a pull request on Github with some changes I have made.