Aliases Marionette as Mn

Resolves #1871.

Potentially breaking if a user has a `window.Mn` variable.
Hmm, `root.Mn` could potentially be different than `root.Marionette`, I think we should update it to support that as well:

```js
var previousMarionette = root.Marionette;
var previousMn = root.Mn;

Marionette.noConflict = function() {
  root.Marionette = previousMarionette;
  root.Mn = previousMn;
};
```