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.

src/sql/hydra.sql

@@ -54,6 +54,7 @@ create table Jobsets (
     errorMsg      text, -- used to signal the last evaluation error etc. for this jobset
     errorTime     integer, -- timestamp associated with errorMsg
     lastCheckedTime integer, -- last time the evaluator looked at this jobset
+    triggerTime   integer, -- set if we were triggered by a push event
     enabled       integer not null default 1,
     enableEmail   integer not null default 1,
     hidden        integer not null default 0,