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.
![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
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).