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, as well as being consumable for reporting purposes.

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
    }
  ]
}
```
Another way to write this is:

```ruby
date = DateTime.parse(params[:date]) rescue nil
```