Add support for prefixes and suffixes on the center ringchart label

Adds support for prefixes and suffixes on the center ringchart label by using `data-prefix` and `data-suffix` attributes on the container element.

## Usage
```html
<div class="ringchart percent" data-value="39" data-suffix="%"></div>
```

## Result
![Clipboard01](https://f.cloud.github.com/assets/1231996/165742/2ba52668-7970-11e2-8a62-80ac78b469fa.png)