Added support for column groups

The column groups in the following image are "Animals" and "Food".
![screen shot 2014-08-06 at 2 16 12 pm](https://cloud.githubusercontent.com/assets/2857274/3832509/2bd3585c-1d9e-11e4-87e7-1986a13eeeb2.png)

To add column groups to the smart-table, use the "column-groups" attribute.
```
<smart-table columns="columns" rows="rows" column-groups="columnGroups"></smart-table>
```

distinguished column groups from columns because it's easier to think about from a data perspective.  It also allows it's own set of features.  For example, it doesn't make sense to apply a cell template to a column group, or to have sorting functionality.