Consider `final` in combination with `protected` to exclude some code analysis scenarios

[Ocramius]

As discussed in person with @asgrim and @akrabat, the following change is not a BC break, because final already makes it impossible to rely on protected properties:

final class Foo
{
    protected $bar = 'baz';
}

changed into:

final class Foo
{
    private $bar = 'baz';
}

This kind of scenario is tricky to represent correctly with the current design of the Comparator

[Ocramius]

Handled in #38