Implemented lfs.chown and lfs.chmod functions

[lch361]

As this library has a function to get file permissions, uid and gid, I think it is a good idea to have a way to set these as well.

[hishamhm]

Thanks for the PR, but this is intended primarily as a portable library. It does leak some Unix-specific info, but at least it does have a minimal implementation of lfs.attributes for Windows. I'm wary about adding new Unix-only functions to the library since that would start a slippery slope which could easily lead it to becoming another implementation of luaposix, which already exists.