Document sendMessage's callback argument

[thatmarvin]

Looks like sendMessage() has an undocumented callback param, which is useful when I need to do stuff after sending the email.

[JonLim]

Hey Marvin,

Thanks for the heads up - my bad! I'll try to fix up the documentation as soon as I can to include the callback parameter.

[edwardhotchkiss]

ping

[JonLim]

Have updated the documentation to show how to use the callback argument, please let me know if you guys need anything else!