Add create_unique_vertex() index method.

Based on [this feature](http://docs.neo4j.org/chunked/stable/rest-api-unique-indexes.html#rest-api-get-or-create-unique-node-create) of Neo4J's REST API.

I actually need this to avoid duplicate nodes in my application. It's my first contribution to this project, so if there's anything wrong with it, please let me know! =)