Facet examples

Add an API to get example documents for each matching facet value.

For example, 

    /unified_search.json?facet_specialist_sectors=10,examples:5,example_scope:global&count=0&filter_organisations[]=hm-revenue-customs

Returns organisation facets for content tagged with HMRC, including an "examples" section in each returned facet, like:

    options": [
      {
        "value": {
          "slug": "paye/statutory-leave-pay",
          "examples": {
            "total": 21,
            "examples": [
              {
                "title": "Statutory Sick Pay: employee fitness to work",
                "link": "/statutory-sick-pay-employee-fitness-to-work"
              },
              {
                "title": "Statutory Maternity Pay: how different employment types affect what you pay",
                "link": "/statutory-maternity-pay-how-different-employment-types-affect-what-you-pay"
              },
              {
                "title": "Statutory Sick Pay: how different employment types affect what you pay",
                "link": "/statutory-sick-pay-how-different-employment-types-affect-what-you-pay"
              },
              {
                "title": "Statutory Maternity Pay: table of dates for employee entitlement",
                "link": "/government/publications/statutory-maternity-pay-table-of-dates-for-employee-entitlement"
              },
              {
                "title": "Statutory Sick Pay: employee circumstances that affect payment",
                "link": "/statutory-sick-pay-employee-circumstances-that-affect-payment"
              }
            ]
          }
        },
        "documents": 21
      },
      ...    

https://www.pivotaltracker.com/story/show/73388400