Adds margin to record sections and panels for sidenav

Closes #647 

* Adds margin to `record-metadata` div that contains metadata panels and sections
* Wraps browse nearby callnumber in smaller screens to accommodate side-nav

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![image](https://cloud.githubusercontent.com/assets/302258/3804575/813f551a-1c28-11e4-85f7-d67aab4b3698.png)
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![image](https://cloud.githubusercontent.com/assets/302258/3804578/90b91abc-1c28-11e4-90eb-108d0a822c35.png)

@jkeck Replaced `768px` with `$screen-sm`