Added a self-executing anonymous function wrapper

I would like to use netteForms.js in my app.js via [grunt-browserify](https://github.com/jmreidy/grunt-browserify) and [shim option](https://github.com/jmreidy/grunt-browserify#shim) like this:
```js
var Nette = require('Nette');

Nette.addError = function(elem, message)
{
    //custom error handling
};
```

[Returning Nette object](https://github.com/o5/bower-nette-forms/blob/master/netteForms.js#L487) is important for me.