Small tweaks to un-bitrot classtut's first example.

Some of these tweaks may change the message that the example is
sending, so this commit is mostly to show a changeset to the fine
people of #perl6 for feedback.
Improve classtut example again

* Bring back private attributes (BUILD submethod needed to bind them properly).
* Add some doc explaining the BUILD usage in the example.
Merge pull request #19 from kanatohodets/master

Fix up classtut example