Feature validation summary

Extends CSVLint to add a "/statistics" page which has both HTML and JSON output summarising validation activity.

To test this out you'll need to create a Summary object by running:

```
rake summary:generate
```

This needs to be added to application crontab to run on a daily or weekly schedule.

I'd appreciate a quick review. I've not added a link from the validations page yet so we can test it against the live data before exposing.

This closes theodi/shared#204