creating directories in a portable way

The command `mkdir -p DIR` isn't available under Windows, so I copied a function [`util.make_dir()`](https://github.com/mnicky/lua-git/blob/master/lua/git/util.lua#L73) that I use in luadist-git and replaced all calls to `mkdir` with it.

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Creating of directories seems to work under Windows now, however, I still get errors when trying to fetch some repository (under Windows). I reported them in issue [#3](https://github.com/mkottman/lua-git/issues/3).