Double backslash in flat object property names on Windows.

Requiring a directory as flat object on Windows machine results in object with double backslash separator in property names. I.e. /blog/index.js -> blog\\\index. That's because the standard Windows directory separator is "\" and it's escaped with another "\". However, "/" is still a valid separator, so I hardcoded sep to "/".

Here is the list of directory separators on different operating systems:
http://en.wikipedia.org/wiki/Path_%28computing%29#Representations_of_paths_by_operating_system_and_shell

Also changed the code style of previous fix to feel more "native" to module. :3