Introduce (private) action

[ro0gr]

This formalizes actions flow by leveraging action utility implemented in this PR.

Here is how an action may look like:

function clickable(selector, queryOptions = {}) {
  return action({ ...queryOptions, selector }), function() {
    const element = findOne(this.node, this.query.selector, this.query);

    return this.adapter.click(element);
  });
}

the action util provide a wrapper for page object descriptor, which is responsible for invocation of user function, and handling error message(if any). That's basically what we did before, but responsibilities were spreaded across different layers a bit.

In the example above you've might noticed this which has:

• node - page object node on which the action is processed
• query - CSS selector and some other (j)query options
• adapter previously known as ExectutionContext, but now it's only responsible for DOM interactions. All The find logic removed from there now.

So this is a reincarnation of execution contexts with a more clear responsibility now.

[coveralls]

Coverage decreased (-0.2%) to 98.418% when pulling 20869f0 on ro0gr:actions into ebac9d3 on san650:v2-beta.