Added chgrp!, chown!, and chmod! to module File

[jdrexler]

Closes Issue #1285

[josevalim]

Thanks @jdrexler! There are a couple fixes pending though.

In particular, Elixir convention for ! is to raise an exception. So we need to have both chmod and chmod!. Take a look at File.rm and File.rm! for an example.

Finally, the tests do not assert the group or the type or the mode of the file after calling such functions, which we must do with the help of File.stat/1.

[jdrexler]

Thanks for the clarification! I will do this fixes soon (probably tomorrow) and send a new pull request.

[josevalim]

Beautiful, thank you!