code: function ( oConfig )
  	{
  		var
  			aoCols = this.s.dt.aoColumns,
  			aColumnsInc = this._fnColumnTargets( oConfig.mColumns ),
  			aColWidths = [],
  			iWidth = 0, iTotal = 0, i, iLen;
  
  		for ( i=0, iLen=aColumnsInc.length ; i<iLen ; i++ )
  		{
  			if ( aColumnsInc[i] )
  			{
  				iWidth = aoCols[i].nTh.offsetWidth;
  				iTotal += iWidth;
  				aColWidths.push( iWidth );
  			}
  		}
  
  		for ( i=0, iLen=aColWidths.length ; i<iLen ; i++ )
  		{
  			aColWidths[i] = aColWidths[i] / iTotal;
  		}
  
  		return aColWidths.join('\t');
  	}
docs: Calculate a unity array with the column width by proportion for a set of columns to be
  included for a button. This is particularly useful for PDF creation, where we can use the
  column widths calculated by the browser to size the columns in the PDF.
  @param   {Object} oConfig Button configuration object
  @returns {Array} Unity array of column ratios