Refactor `tail` function

[leonderijke]

Tail now returns the rest of the list instead of the last item. It is a common idiom to split a list into a head (first item) and tail (rest of the list), especially when using recursion. This change supports that use case.

Fixes #25

[KittyGiraudel]

Excellent, thank you very much! I'll write the docs and release a new gem version when I get a chance. :)

[leonderijke]

Awesome, thanks!