Introduce nodeset method.

It was a pain to keep figuring out if we we're dealing with a `Node` or `NodeSet`. So polymorphism :smile:

There might be a problem if users ever pass in an array of elements (instead of a `NodeSet`).
For instance using an `Enumerable` method like `map` returns an array.
Which could confuse people... but then again why would they do that, when they already have `assert_select` to pick out elements?

Which is why I've removed the `is_a?(Array)` checks. It's easy to make sure we have a node set when needed.

@rafaelfranca