Go to http://getbootstrap.com/customize/ and toggle just what you want from the BS3 framework and then click “Compile and Download” and you’ll get the CSS and JS that you chose.
Open up the CSS and remove all but the grid. They include some normalize stuff too. And you’ll need to adjust all the styles on your site to box-sizing: border-box – http://www.w3schools.com/cssref/css3_pr_box-sizing.asp