Chainable methods

Hi again,

It's duplicate of pretty old #244, but hopefully this time PR is more suitable for [contribution](https://github.com/eternicode/bootstrap-datepicker/blob/master/CONTRIBUTING.md) ;-)

So again:

I get an idea (, and need ;) ) to chain datepicker methods. E.g.
```javascript
var myDatepicker = $('.selector').data('datepicker')
                                    .setDate(defaultDate)
                                    .show()
```