exported csv adheres to table td colspans

Before, if a .ng-table-group column had colspan > 1, it would always align left in the csv export and leave blank columns to the right.  This makes the export adhere colspans in the export.

![screen shot 2014-08-07 at 4 06 41 pm](https://cloud.githubusercontent.com/assets/186677/3848296/80abbd6c-1e6e-11e4-9a31-7261f6520fef.png)