Enables jQuery object to nextSelector and prevSelector

###### What is does?

Adds support to jquery objects on nextSelector and prevSelector.

```js
$('.bxslider').bxSlider({
  nextSelector: $('#slider-next'),
  prevSelector: $('#slider-prev')
});
```

```html
<ul class="bxslider">
  <li><img src="/images/730_200/hill_trees.jpg" /></li>
  <li><img src="/images/730_200/me_trees.jpg" /></li>
  <li><img src="/images/730_200/houses.jpg" /></li>
</ul>

<div class="outside">
  <h3>This div is outside of the slider</h3>
  <p><span id="slider-prev"></span> | <span id="slider-next"></span></p>
</div>
```

###### Why?
You can now easily dynamically manipulate sliders with custom controls.