Made DataTable layout independent of Bootstrap's grid column count

For our project we're using a customized version of Twitter's Bootstrap (http://getbootstrap.com/customize/). For better UI handling we changed the grid column count (@grid-columns) from 12 to 60 (which is quite common). The DataTables use the hard coded .col-xs-6 style for creating a 50% wide column - which breaks the layout for customized Bootstraps.

With this pull request the columns are created independently of the total grid column count.