Fix/mail transports

/to @raymondfeng @bajtos 

This provides a sane default for the mail connector.

Given the following example you will see a warning. Maybe it should just throw. If that is the case we should provide a "testing" transport similar to the memory connector that will work out of the box with a new LB app.

```js
var email = require('app').models.email;

email.send({
  to: 'joe@bob.com',
  from: 'bob@jim.com',
  subject: 'hey man'
}, function (err, msg) {
  console.log(msg);
});
```

```sh
# output
Warning: No email transport specified for sending email. Setup a transport to send mail messages.
{ to: 'joe@bob.com', from: 'bob@jim.com', subject: 'hey man' }
```