Fixed: CSS for generated sprites have image URLs containing paths of temporary tree folders

Hi Aboubakr,
I left the API as is, no extra options. So what we discussed is basically done without the need to supply any extra options like vendor directories. What happens now is:
1. Generate everything in the `srcDir`.
2. Copy over all that is relevant to the `destDir`, which is basically everything except original sass files and the cache. *(It might pay off to actually leave the cache if it can be reused. I'm actually pretty unfamiliar with compass. It's that they use it at my new job that needs a build script.)*
3. Sanitize the `srcDir` by removing sass cache and css files.

I added a few tests that primarily target steps 2 and 3. The fixture directory contains some fake or small tests. Any comment or improvement is more than welcome.

Greetz,
Ferry
I am for keeping the sass-cache directory, it would be great if we add an option `keepCache` to be more flexible!