Add a plugin mechanism

You can now add plugins to Hydra by writing a module called
Hydra::Plugin::<whatever> and putting it in Perl's search path.  The
only plugin operation currently supported in buildFinished, called
when hydra-build has finished doing a build.

For instance, a Twitter notification plugin would look like this:

  package Hydra::Plugin::TwitterNotification;

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

  1;
Use "can" to check whether a plugin supports buildFinished
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}.
  }