Added schema for a com.mandrill/recipient_unsubscribed event (closes #44)
Added schema for a com.mandrill/message_sent event (closes #36)
Added schema for a com.mandrill/message_soft_bounced event (closes #41)
Added schema for a com.mandrill/message_opened event (closes #38)
Added schema for a com.mandrill/message_marked_as_spam event (closes #39)
Added schema for a com.mandrill/message_delayed event (closes #40)
Added schema for a com.mandrill/message_clicked event (closes #42)
Added schema for a com.mandrill/message_bounced event (closes #37)