We now also use it from wkd and it seems like a generally
useful kind of response.

We were using Time.at(expirationdate) even if it was nil which
led to using the Time.at(0). Instead an unset expirationdate
is meant to not expire the key at all.

Our tests did not catch this because the assertions were in
blocks that did not get run at all. (at least in the HKP
integration test).

using an array of symbols representing the different checks now.

Still needs something better than all these elsifs though

This way the other RequestHandlers can give it a try.
If none handles it we'll get a 404 anyway. But maybe there's
been an exception before so a 502 should be send.

- add fingerprint unit tests
- add integration test
- implement by_fingerprint

This became possible because we now use celluloid.
Celluloid handles asynchronity without the need for callbacks
or blocks.