Book: Add media queries in the CSS.

Add `@media` to change the layout when the screen size is <= 320px. The current layout looks too squished.

Old:
![ios_iphone_5s 1](https://f.cloud.github.com/assets/81969/1484203/cd86609e-4703-11e3-8469-bcf50cc14f99.jpg)

New:
![ios_iphone_5s](https://f.cloud.github.com/assets/81969/1484205/d0a0017c-4703-11e3-83d4-297915cd7ea7.jpg)
![win7_ie_9 0](https://f.cloud.github.com/assets/81969/1484206/d2287b1e-4703-11e3-858f-8ceb74aa23b6.jpg)
![android_lg_nexus_4](https://f.cloud.github.com/assets/81969/1484207/d3ed812e-4703-11e3-9aba-79a3a5d88c98.jpg)