Added chunked sending for 500+ messages

Sending 500+ messages at once now creates chunks so Postmark won't complain. This fixes issue #20. The only downside is the response changes to an array above 500 messages, I think this could be confusing. Should we change all responses to arrays or should the 500+ messages response be converted to some kind of string? What do you think would be right withou confusing too much users?