Fix `.col-*-12` doesn't get floated on purpose--there's no need since it's full-width

Created by: kmukku

Like said before on posts #10152 (closed) and #9454 (closed) full-width columns (.col-*-12) should also be floated, since those are not inline elements.

As other columns are floating, full-width column will ignore those because browser removes floating elements from the normal flow. For that reason full-width element will be displayed just where it occurs in the text, and that is under floated columns, not next to those.

Inline elements inside full-width column do respect the boundaries of the floating elements, but that is not enough.

See this fiddle: http://jsfiddle.net/8Bg2H/ and remove the "float: left;" of the full-width element to see what I mean.

This change will affect to files grid.less and mixins.less (.make-grid-columns-float).