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}.
}