Add labels for use in filters and on service pages

_This commit is a *proposal* of content structure. It modifies one page of the content as an example. It is backwards compatible with all current consumers of the content._

We now have the requirement to store this additional information:
- **Service page: An appropriate label for the response the supplier gave to each question in the SSP, for example:**
  ![screen shot 2014-12-30 at 15 24 08](https://cloud.githubusercontent.com/assets/355079/5579789/0450fb42-9038-11e4-9a6d-7a980b400057.png)
- **Search results page: A label for each option in a set of filters, for example:**
  ![screen shot 2014-12-30 at 15 25 31](https://cloud.githubusercontent.com/assets/355079/5579797/27544018-9038-11e4-9d2a-eaf0fca9bbf2.png)
- **Search results page: Which of the options within the filters are presented to the user, for example we probably don't need to let users filter by 'none of the above' here:**
  ![screen shot 2014-12-30 at 15 26 25](https://cloud.githubusercontent.com/assets/355079/5579806/5468e93c-9038-11e4-9d00-e8de45f0f123.png)

This commit adds two additional attributes for each question's `field` object:
- `filterLabel`: The label that will be shown alongside the checkbox when filtering
- `servicePageLabel`: The label that will be shown before a supplier's response on a service page

If `filterLabel` or `servicePageLabel` are omitted then the _intention_ is that the respective filter or response should not be rendered to the page. The implementation of this is left to the application consuming the content.