Include duplicate_of in need search results

Without the `duplicate_of` field Maslow’s search results table misrepresents duplicate needs by showing an empty “Decision made” column.

Fixes https://www.pivotaltracker.com/story/show/78193142

Before:
![screen shot 2014-10-03 at 14 17 19](https://cloud.githubusercontent.com/assets/319055/4506281/08190a0c-4b01-11e4-9654-f8a0ea378592.png)

After:
![screen shot 2014-10-03 at 14 17 12](https://cloud.githubusercontent.com/assets/319055/4506285/0e92fbb8-4b01-11e4-912f-a902af318c19.png)
String is totally fine since the only operation that'll be done on this is looking up exact values.  Integer types are indexed to allow for efficient range and sort operations, which would be just a bit of a waste here (though not a problem either).