Adding rn-carousel-control for next and previous buttons, improving indicators and CSS

This may be too dense a pull request for your liking... I'm happy to break it up a bit, but this was kind of a natural progression in the development of something I'm working on lately so I figured I'd get this to you for feedback.

### Carousel controls (next and previous)
The most notable change is the addition of the ```rn-carousel-control``` directive, which, like the ```rn--carousel-indicator```, adds controls for the user to progress forwards and backwards through the carousel. The back button is hidden when on the first slide, and the next button is hidden on the last slide. It could be written to loop if desired - perhaps that could be an attribute in the future as well.

```html
<ul rn-carousel rn-carousel-control>
  <li ng-repeat="item in items">{{ item }}</li>
</ul>
```

### Sass and the use of CSS content attribute
I also took the liberty to make a few changes to the existing setup. First, I think that using a CSS preprocessor would be wise, as people will be able to copy the SASS file into their projects with complex styles and build the CSS to their liking. It also will allow for greater customization.

To that end, I also changed the template for indicators to not include the "•" character directly anymore. Instead, I think it would be easier for someone to place custom text in through the CSS content attribute. That way, it can be overwritten in a variety of ways through CSS.

```css
.rn-carousel-indicator:before {
  content: "custom buttonz";
}
```

I implemented rn-carousel-control the same way. In my current project, I'm overriding the default caret symbols with the carets from [Font Awesome](http://fontawesome.io) using only CSS.

![screenshot 2014-01-20 14 59 50](https://f.cloud.github.com/assets/231133/1958367/553f98bc-8218-11e3-9278-ce2badbcc9d7.png)