AbstractRelated should set names for child rule

[henriquemoody]

A code like this:

try {
    v::key('username', v::alnum()->noWhitespace()->length(4,22))->check($_POST);
} catch (ValidationExceptionInterface $exception) {
    echo $exception->getMainMessage().PHP_EOL;
}

Should display messages like this:

username must not contain whitespace

Fix #86

[henriquemoody]

I'm not sure if it's a bug fix or a behaviour change.

If it is really a bug fix, it's necessary to cherry-pick 7f8815e to 0.8 and 0.9 branches, then create a PATCH versions.