This commit fixes relative path issue (#4) by passing the file name with...

... relative path to the compass.js lib callback

This is a simple solution to this relative path issue. It just creates a new variable called ```relative_file_path``` containing the file name with path relative to sass directory.

Examples (considering that sass dir is ```sass```):

Folder | relative_file_name
--- | ---
sass/base/screen.scss | base/screen.scss
sass/base/components/buttons.scss | base/components/buttons.scss

In the end, the callback is called with ```relative_file_name``` instead of ```file_name```, this way there will be no exception thrown, solving the error described in (#4).