Added support to change/retrieve onInit and onSwitchChange through metho...

Accordingly the docs:

"In Bootstrap Switch, every option is also a method"

However there aren't methods for the options onInit and onSwitchChange, so, I created them and now we are able to perform a call like this to set a callback:

```
$(element).bootstrapSwitch('onSwitchChange', function(event, state) {} ); 
```