add flex-flow to multi-up grid

Playing with some responsiveness:

![screen shot 2014-02-21 at 17 34 50](https://f.cloud.github.com/assets/111218/2236551/a7ef014e-9b61-11e3-8d8c-1c8def4e6e43.png)
![screen shot 2014-02-21 at 17 35 05](https://f.cloud.github.com/assets/111218/2236549/a7da5258-9b61-11e3-9f2b-1e43f093ca61.png)
![screen shot 2014-02-21 at 17 35 12](https://f.cloud.github.com/assets/111218/2236550/a7db8b64-9b61-11e3-96d5-afaa0a244337.png)