Feature/view for valid certs

We now store the expiry with certs in place of the creation date. I think this is more flexible - but your choice might differ. I'm happy to revert that decision.

Also move complex identity views into js designs.
Includes test.

Here's how you would query it from outside rails:
```
$ curl
'localhost:5984/identities/_design/Identity/_view/cert_fingerprints_by_expiry?startkey="2014-07-05"'
{"total_rows":4,"offset":1,"rows":[
{"id":"6c9091d4f13eaeaa6062c9d0528fd34d","key":"2014-07-05","value":"fingerprint"},
{"id":"6f3aa93828b4f6978d551f2623b9d103","key":"2014-07-05","value":"fingerprint"},
{"id":"b6cafacfa65042679691cd5065fb19e3","key":"2014-07-07","value":"fp"}
]}
```

Note that the expiry will be used as the key. So you should use the current data (or yesterday) as the startkey to get all fingerprints that have not expired yet.

The fingerprint itself is in the value. No need to include docs.