added email custom headers in user verify

I some cases is necesary to add custom header in user.verify, for example to track an email
Please simplify this function and keep only checks relevant for this test.

I would also prefer to check that the e-mail message contains the correct header, to make sure we are calling the nodemailer API correctly.

Example:

```js
user.verify(options, function(err,result) {
  if (err) return done(err);
  var msg = result.email.response.toString('utf-8');
  assert(~msg.indexOf('message-id: custom-header-value'));
  done();
});
```

@ritch  you are probably more familiar with `Email.send` API. What's the best way to implement this check?