Wrap use of posixpath in an utility function

[nOOb3167]

Implement utility function, as suggested by comment on pull request #93.

[shevron]

Thanks, this is great. One small comment is that I'd rename the function to something more explicit like join_path to avoid confusion with general purpose str.join and the likes. join is a very generic name, and future me would surely get confused seeing it all over the code.

[rufuspollock]

@nOOb3167 i know this has been some time - we'd still happily have this in if modified as per comment. If you are now too busy just let us know and we'll look to add this ourselves at some point.

[demenech]

Looks good. Will keep an eye on it, those conflicts seem simple to fix.

CC: @anuveyatsu