Add SMTP ticker interval

Add SMTP ticker interval that will set a minimum time interval between each email.

If more than one message is received in the period, the mails are concatenated.

I was unable to tun the tests on my system, and the number of newlines may be different to what it expects. The additional newline was added to separate different joined email bodies.
>Heka supports Linux, OSX, and Windows, separator should vary across platforms. 
Return+Newline was used in the original implementation, so I didn't want to change that.

strings.Join seems like a very good way of reducing garbage!