Mailconnector transports issue460

This commit will resolve the issue #460 regarding using single or multiple transports within the config file.  I have tested with the existing mocha tests and all are passing.  This PR does not resolve issue https://github.com/strongloop/loopback-boot/issues/4 as the `user` and `pass` properties that are part of the nodemailer transport object are not environment variables. 
Modifying objects passed as function arguments is a bad idea. Please rewrite it in a way that will not modify the arguments.

Example:


```js
var transports = settings.transports;
if (!transports && settings.transport) {
  transports = [settings.transport];
}

if (!transports) {
  transports = []
}
```