`create` methods use params

This makes node and rel creation use params in their Cypher `CREATE` clauses. Not only will it improve performance, especially in busy environments, but it protects against cypher injection during create.

I also couldn't help myself and did a little refactoring while in the process of figuring out how it worked and how to fix it. I'm gunning for unnecessary variables and object creations.

Corrects https://github.com/neo4jrb/neo4j/issues/612.