potential fix for sprite-path compass bug

Potential workaround for a bug in sprite-path() in compass 0.12.2. 

Issue:
https://github.com/pierreburel/compass-hdpi/issues/3

If using a generated images folder defined in config rb. Example:
```
generated_images_dir = "images/generated"
```

You get an error such as this:

```
 No such file or directory - {path to site}/images/icons-s66517bab07.png
```

Where it should be looking in:

```
{path to site}/images/generated/icons-s66517bab07.png
```

In this workaround the user would have to set a variable for the generated image path. There is probably a more elegant way of doing this.