Address Early Adpoter's feedback

This PR addresses all of the points in the feedback as discussed in #8, for @honzajavorek to review & merge
there's no class `elements`, it's called `group`

by the way, I always struggle to realize when this class gets used... I think I found out once, but I forgot it probably. `One of` itself is array of `Element`s, because it inherits from `Elements`. It's array of mutually exclusive elements, so in case of e.g. "one of two properties" this tree node will be an array of two elements with class `property`. The same applies for `mixin` and nested `oneOf`s. I get that. But  then, what is the `group` for?

We should make clearer in our wording where the use case for need to have `group` comes from. The rest is pretty straightfoward, but this is always a bit of mystery for me and I have to think about it very deeply.