Pass the correct callback to dispatchMail

[smurthas]

If several messages got queued up during the fs.readFile call for a single template, when the template was finally loaded, dispatchMail would get called for each message, but would get passed the callback associated with the first one (the only one that invoked the fs.readFile call). This bundles the callback in with the message itself, ensuring that the callback associated with that message is the one passed to dispatchMail

[smurthas]

Do you think you'll be able to pull this in anytime soon? We've been pointing to my fork in our package.json for a while now. Thanks!