Show summaries on results for Drug Safety Alerts

This work checks the document_type against a list of formats that we want summaries listed for. [Ticket](https://trello.com/c/3tM31WOu/279-dsu-finder-add-summary-to-each-result-1).

![screen shot 2014-08-28 at 13 52 45](https://cloud.githubusercontent.com/assets/449004/4075682/9878ff46-2eb2-11e4-8f00-d06d37224e11.png)
It would be better to solve this by using the existing specialisations of `AbstractDocument` – in this case, `DrugSafetyUpdate`.

When you see this kind of inspection of a "type" or "kind" or similar field to determine what to do, it's usually a sign you could be using some kind of specialisation of objects.