non float: left on col-*-12 cause overlap of elements when the number of columns in a row add up to > 12

Created by: bassjobsen

Example: `

.col-xs-6 .col-md-4

.col-xs-6 .col-md-4

.col-xs-6 .col-md-4

`

see: http://stackoverflow.com/questions/18428450/bootstrap-3-layout-with-panel-display-not-correct-on-small-device/18433782