Examples:
Wisper.subscribe(AuditRecorder.new)
Wisper.subscribe(AuditRecorder.new, StatsRecorder.new)
Wisper.subscribe(AuditRecorder.new, on: 'order_created')
Wisper.subscribe(AuditRecorder.new, scope: 'MyPublisher')
Wisper.subscribe(AuditRecorder.new, StatsRecorder.new) do
# ..
end