Toggle value method

As there is currently no method to easily select or deselect a value based on its current state I implemented a toggleValue feature.

```
$('.myselect').selectpicker('toggleVal', 'myvalue');
```

The example selects "myvalue" if it is not already selected and deselects it in case it its already selected.

A major benefit of adding this to the API is that it supports the change event, that is triggered by the plugin itself.