Code Review: Done

Hey Markus,

I just finished the code review of this gem. Everything's nice and dandy :+1:
However: I tweaked the README a little and adjusted some comments/documentation. So here is a little pull request. No code was changed.

Two questions:

## Nested class

In the Operation N-ary class you have a nested class. See [here](https://github.com/mbj/aql/blob/master/lib/aql/node/operation/nary.rb#L33-51). I've never seen that before. Why did you decide to go that route?

## Classes inheriting self

Why do some classes inherit `self`? I've never seen this before. What is the reasoning behind it?