* Add Request class with HTTP signature generator

Spec: https://tools.ietf.org/html/draft-cavage-http-signatures-06

* Add HTTP signature verification concern

* Add test for SignatureVerification concern

* Add basic test for Request class

* Make PuSH subscribe/unsubscribe requests use new Request class

Accidentally fix lease_seconds not being set and sent properly, and
change the new minimum subscription duration to 1 day

* Make all PuSH workers use new Request class

* Make Salmon sender use new Request class

* Make FetchLinkService use new Request class

* Make FetchAtomService use the new Request class

* Make Remotable use the new Request class

* Make ResolveRemoteAccountService use the new Request class

* Add more tests

* Allow +-30 seconds window for signed request to remain valid

* Disable time window validation for signed requests, restore 7 days
as PuSH subscription duration (which was previous default due to a bug)

* Framework for delivery worker spec

* Refactor of pubsub delivery worker

* Add specs for valid challenge response from pubsub confirmation worker

* Refactor the pubsub confirmation worker