Send ETag and Expires headers
hydra-evaluator: Always pick the jobset that hasn't been evaluated longest
Fix duplicate function name
Handle the case where a jobset has never been evaluated
Support push notification of repository changes

External machines can now notify Hydra that it should check a
repository by sending a GET or PUSH request to /api/push, providing a
list of jobsets to be checked and/or a list of repository URLs.  In
the latter case, all jobsets that have any of the specified
repositories as an input will be checked.

For instance, you can configure GitHub or BitBucket to send a request
to the URL

  http://hydra.example.org/api/push?repos=git://github.com/NixOS/nixpkgs.git

to trigger evaluation of all jobsets that have
git://github.com/NixOS/nixpkgs.git as an input, or to the URL

  http://hydra.example.org/api/push?jobsets=patchelf:trunk,nixpkgs:trunk

to trigger evaluation of just the specified jobsets.