Return pure CSS instead of JavaScript

The current sass-loader generates `module.exports = '..css..'`. This makes using the css-loader impossible because it will try to parse it as CSS.

According to @sorka's [guidelines](http://webpack.github.io/docs/how-to-write-a-loader.html) a loader should do only one thing. I've adjusted the sass-loader to only transform the sass string to a css string. If you like to just retrieve a css string, you now need to use it in conjunction with the raw-loader:

```javascript
var css = require("raw!sass!./file.scss");
```

I've also updated the README accordingly. The sass-loader now works exactly like the less-loader.

This is a breaking change and should receive at least a minor version bump (we're still in development, so it's ok not do a major version bump).