Update events in pattern library to use grid markup

Update events in pattern library to use new grid markup.

![screen shot 2014-12-30 at 15 48 14](https://cloud.githubusercontent.com/assets/123386/5580013/fa55f38c-903b-11e4-9f22-a2b7ecfe83f2.png)
![screen shot 2014-12-30 at 15 51 03](https://cloud.githubusercontent.com/assets/123386/5580014/fc06407e-903b-11e4-829f-e0bc80b9a704.png)

Also fix a layout issue with hero events highlighted by the pattern library where backgrounds were overflowing. This didn't show on the main events list as the container has `overflow:hidden`, but it's not great to rely on `overflow:hidden` to 'solve' layout issues.

Before:
![screen shot 2014-12-30 at 15 47 17](https://cloud.githubusercontent.com/assets/123386/5580024/3876d0aa-903c-11e4-9cfc-665d1223ad1d.png)

After:
![screen shot 2014-12-30 at 15 47 51](https://cloud.githubusercontent.com/assets/123386/5580026/3be09456-903c-11e4-88ae-bf56f06b3394.png)