Expose problem report daily totals for a given path

eg

```
{
  artefact: [...],
  performance: {
    page_views: [...],
    searches: [...],
    problem_reports: [
      {
        timestamp: "2014-08-20T00:00:00Z",
        value: 3
      },
      {
        timestamp: "2014-08-21T00:00:00Z",
        value: 2
      },
      {
        timestamp: "2014-08-22T00:00:00Z",
        value: 15
      }
    ],
    search_terms: [...]
  }
}
```