Improved Windows path support in lessc

[chrizel]

The include-path implementation in the current lessc version doesn't work very well with Windows, because it doesn't recognize absolute Windows paths and uses ":" as path separator which won't work on Windows because ":" is part of an absolute path there.

This commit improves the lessc windows compatibility in the following ways:

• If the os type is Windows, it uses ";" as include-path separator instead of ":"
• The code uses path.resolve instead of recognizing and joining absolute paths itself

[cloudhead]

Have you tested this on linux or os x? Just want to make sure it doesn't break anything.

[chrizel]

I've tested it under OSX and it still works as expected. Multiple include-paths separated by ":" still work with absolute and relative paths.