added throttling (for AWS usage) and tested in SilverStripe 2.3.0

Hi there,

I was in need of an SmtpMailer for my SilverStripe 2.3.0 application. Also, I'm porting my SilverStripe application into Amazon and using their SES for e-mails. However, there is throttling going on there and to ensure that some scripts that may be using $email->send() in a loop, I wanted to ensure that there was the ability to control the speed with which e-mails are sent.

I've tested this and Amazon didn't kick back any responses as long as I stayed within my limit, which happens to be 5 e-mails/second.

I'd appreciate you accepting my changes.

Thanks!

Nick