Add problem report totals endpoint

This endpoint allows getting summaries by day, which is useful
because it allows publishers to see which part of the site to focus
on and improve.

Request:
```
GET /anonymous-feedback/problem-reports/2014-09-24/totals
```

Response:
```
{
  date: "2014-09-24T00:00:00+00:00",
  results: [
    {
      "path": "/tax-disc",
      "total": 3
    },
    {
      "path": "/jobmatch",
      "total": 2
    }
  ]
}
```
Merge pull request #17 from alphagov/add-problem-report-totals-endpoint

Add problem report totals endpoint