Naive API endoint for creating stats for a given survey

Use case: we want to provide basic overview information for any given survey. Many surveys are too large to efficiently do this in the browser. 

This adds an endpoint at `api/surveys/:id/stats` that returns counts for each answer, like so:

  {
    survey: {
      dumping: {
        yes: 8,
        no: 9
      },
      fire-damage: {
        yes: 6
      },
      condition: {
        good: 2,
        poor: 3,
        fair: 1,
        excellent: 3
      },
      ...
    }
  }

Known deficiencies: 
- No caching. The data is recalculated on each request even if it hasn't changed
- Doesn't include stats on response times, users
- Data format may not be the best, depending on how we decide to consume the data (goal: no data transformation necessary on the client)