Adding events for fadeIn and fadeOut

Adding options to add event when a toastr fades in or fades out.

Adds two new options to the list: onFadeIn and onFadeOut, both are function.

An example of usage would be:

```javascript
var onFadeIn = function () { console.log("Message"); }
toastr.success('One', 'Two', { onFadeIn: onFadeIn });
```