Add possibility to import from multiple pathes

[damaddin]

Merged the code of frido37 and added the possibility to define multiple pathes for resolving imports in less files. We required this for our whitelabelling solution to generate multiple .css files, cascading and falling back to a master directory.

[sandroboehme]

@damaddin: The fixes create merge conflicts that I cannot fix. Please tell me if you want to and have time to fix them for the upcoming release.