Add sourceUrl when debug is true

Setting debug `true` before `load`:
```js
Ractive.defaults.debug = true
Ractive.load().then( function () {
    ...
```
will add `//# sourceURL=<component-link>.js` to the end of the script, causing them to appear in Sources for debugging:

![image](https://cloud.githubusercontent.com/assets/478864/2737585/84e05d6e-c67d-11e3-895e-0da547e3444a.png)