Add Enum.group

Enum lacks a function for slicing a list up into equally sized sublists. I had need of such a function so I wrote it. I originally called the method `group` as given here, but now I think `slice` might be better, but I leave that up to others to decide.

NOTE: Since I am new to Elixir, I do not pretend that this pull request is as polished as it needs to be. But I think this function is a needed one and my code is far enough along that I require feedback in order to be able to finish it. Thanks.