Added option for output directory

This adds an options to change the path the compiled files are written to and are served from. The path from which is read is still the same as before.
It's useful if you don't want to mess your directory with `asset-*`-files up and also want to prohibit that your original files are served.

```javascript
var assets = ferguson('./assets', {
    outputDir: './compiled'
})
```

The manifest-file still remains in the read directory.

This pull request isn't finished yet, I want to add tests to it (but I'm not sure how to deal with that) and there's a bug where old compiled files aren't cleaned up. I don't know if this comes from my changes.