* 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)
