Include document_collections in the main nav so publications is active

At the moment nothing in the nav is in the active state when viewing a document collection.

A collection can contain many document types, so perhaps this is the expected behaviour but it looks odd. I have defaulted to use the publications type to be active as most collections contain publications.

Before
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![screen shot 2014-03-17 at 17 19 27](https://f.cloud.github.com/assets/215/2438977/5a1aaf00-adf8-11e3-9085-03ff2d1c5005.png)
After
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![screen shot 2014-03-17 at 17 19 31](https://f.cloud.github.com/assets/215/2438978/5f40855e-adf8-11e3-9df5-bd03ae57d7c5.png)