Add content-block (sir-trevor block class) to all custom blocks and add top/bottom margin to it.

Closes #529 

#### Carousel
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![carousel-block](https://cloud.githubusercontent.com/assets/96776/2650816/203fc9ee-bf7a-11e3-81de-f730be3c630d.png)

#### Stacked Widgets
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![many-widgets](https://cloud.githubusercontent.com/assets/96776/2650856/0e3aa0ce-bf7b-11e3-93c3-afb0db7c78a3.png)