Add support for card rows to card grid helper, add column hide classes

As it says. Adds support for columns that can 'drop off' - primarily to be used when cards are displayed in rows on larger screens.

Specs green.

![screen shot 2014-11-28 at 14 03 57](https://cloud.githubusercontent.com/assets/271645/5228828/73f4b83e-7707-11e4-9734-24bf96f01782.png)
![screen shot 2014-11-28 at 14 04 03](https://cloud.githubusercontent.com/assets/271645/5228827/73f1bcd8-7707-11e4-8647-a1bf76a464cd.png)