Replace @while loops with @for loops
Replace @while loops with @for loops

This makes the code shorter and easier to read. In addition, it makes it
compile successfully with pyScss, which doesn't support @while loops.

pure-zero/sass-twitter-bootstrap,pure-zero/sass-twitter-bootstrap,jlong/sass-bootstrap-defunct,jlong/sass-bootstrap-defunct,thanj/quilt-next,thanj/quilt-next,pure-zero/sass-twitter-bootstrap