Use OO-style plugins

This is mostly so we don't have to pass around common parameters like
"db" and "config", and we don't have to check for the existence of
methods.

A plugin now looks like this:

  package Hydra::Plugin::TwitterNotification;

  use parent 'Hydra::Plugin';

  sub buildFinished {
      my ($self, $build, $dependents) = @_;
      print STDERR "tweeting about build ", $build->id, "\n";
      # Send tweet...
      # Hydra database is $self->{db}.
  }

src/lib/Hydra/Plugin.pm

@@ -1,14 +1,23 @@
 package Hydra::Plugin;
 
+use strict;
 use Module::Pluggable
     search_path => "Hydra::Plugin",
-    require     => 1;
+    instantiate => 'new';
+
+sub new {
+    my ($class, %args) = @_;
+    my $self = { db => $args{db}, config => $args{config} };
+    bless $self, $class;
+    return $self;
+}
 
-# $plugin->buildFinished($db, $config, $build, $dependents):
-#
 # Called when build $build has finished.  If the build failed, then
 # $dependents is an array ref to a list of builds that have also
 # failed as a result (i.e. because they depend on $build or a failed
 # dependeny of $build).
+sub buildFinished {
+    my ($self, $build, $dependents) = @_;
+}
 
 1;