Orbit: Allow a configurable selector for slides

### Context
When working with [Ember.js](http://emberjs.com/), the library generates [metamorphs](http://emberjs.com/guides/understanding-ember/keeping-templates-up-to-date/) to support the features it provides. These tend to go everywhere, and js libraries that aren't careful about what they select often pick up these `script` tags by mistake.

### Problem
Here is a screenshot from a project I'm working on. This Orbit slideshow is supposed to have 3 items, generated dynamically, however it is generating 9 slides due to metamorphs inserted in the dom by Ember.
![screen shot 2014-01-07 at 1 52 55 am](https://f.cloud.github.com/assets/50039/1857868/87b9b044-7781-11e3-9aa7-e05b03fc07f9.png)

### Solution

```javascript
$(document).foundation({
  orbit: {slide_selector: 'li'}
});
```

My first instinct was to simply hardcode a selector for the slides, `li` seemed appropriate. But with only slightly more effort I added a configurable selector so that developers could make it whatever they deem appropriate. By default, the selector is "*", making Orbit retain the same behavior as today (backwards compatibility).