Fix some issues in Windows

This fixes a couple of issues that currently prevent certain parts of sass-graph from working properly on Windows:

1) When importing a mixin, the regex that prepends "_" to the filename expects only forward slashes in the path. Replacing the regex with path methods makes it work regardless of which directory separators are used.
2) node-glob always returns paths with forward slashes, this leads to duplicate entries in the resulting imports in Windows. Using node's path.resolve makes the directory separators consistent.