Make this plugin more Windows-friendly by fixing several issues.

Without this change Sublime Text is rendered broken shortly after Dart package installation.

What happens is that dartlint.py writes Windows path with backslashes in color_scheme property during initialization(in check_theme), but then sublime.load_resource(theme) can't work when those backslashes in theme path.

The fix is to force forward slashes in the path by using posixpath even on Windows.