Fixed a wrong URL bug.

By default, gears will generate a hash for all files in URL attribute in CSS files.
But bug appears when server run on Windows platform.
Here is the introduction to the bug:
Bug in file: https://github.com/gears/gears/blob/develop/gears/processors/hexdigest_paths.py#L43
```python
    ....
    return os.path.relpath(asset.hexdigest_path, self.current_dir)
```
the function return a string(not raw string), it will output "\\" with "\". So the browser get return like:
```
#something {
    background: url('..\images\logo.b27b3c7f0a92855c976a0fef707483d3169151c3.png');
}
```
and it will visit: `..imageslogo.b27b3c7f0a92855c976a0fef707483d3169151c3.png`.

To fix this bug, just convert __string__ to __raw string__.