This capability -- adding an *existing* node to an index unless one already
exists -- has been removed in 1.9. Sad day. =/
Thanks @flipside for both contributing this implementation and then giving a
heads-up that this is no longer possible.
Closes #14 -- we're going to have to wait til Neo4j 2.0's new indexing
framework, which will hopefully re-enable this (ideally via Cypher).