Fix Issue #110: Can't dump circular data

Use YAML aliases to solve circular references. Assign to every
object and list an alias. This way it also maintains
the uniqueness of the objects.

The solution is not the most elegant (assigns a new attribute to
every object and then delete the attribute, and every object has
and alias even if they are not referenced again). Either way, it
works and does not suppose much oveload.

The file test/issues/0068.js is modified so it can expect the
solution including the aliases.