Support File.chmod/2, File.chgrp/2 and File.chown/2

[josevalim]

And their ! variants: File.chmod!/2, File.chgrp!/2 and File.chown!/2. They could be implemented simply using the functions available in the Erlang :file module.

[alco]

@yrashk has also mentioned make_symlink (if we're already adding platform-specific functionality).

[josevalim]

We can tackle symlink on another issue (probably call it ln_s keeping the UNIX naming aspect). I think it is ok for us to add platform-specific stuff, Erlang already has those and we just need to keep the same semantics.