Feature: Built-in system for converting from query data

[JasonTheAdams]

Resolves #15

As noted in the Issue, models didn't have a first-class way of building from query data, which was a problem since relationships relied on this.

This PR introduces the ability to build from query data right into the Model class. For models that only have primitive properties, no extra work is needed. If the model has class properties, then the way to convert them is by overloading the castValueForProperty static method. It will probably look something like this:

protected static function castValueForProperty( string $type, $value, string $property ) {
	switch($type) {
		case: DateTimeInterface:
			return new ImmutableDateTime($value);
		default:
			 // let the parent handle the primitives
			return parent::castValueForProperty($type, $value, $property);
	}
}

I decided to leave (although rename) the ModelBuildsFromQueryData interface. I'm really tempted to get rid of it. But I wanted to defer to @borkweb, who first added it, as to whether he thinks it's still necessary. I can't personally think of a use-case where we'd want to pass an interface whose only purpose is to be able to create models, when the underlying model is what's actually concretely passed. That sounds more like a factory.