meant to move id -> @id, also turned identity in the test titles into
@identity.

we compare the cert that expires last to the one we just saved. So we need to make sure the one we saved is the one that expires last.

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.

Identity.new.valid? should not crash. So validate presence where needed and
skip the other validations if the value is absent.

errors.each iterates through all errors for all attrbibutes nicely.

hash token with sha512 against timing attacs #3398