Transactional Endpoint & batch Cypher queries

Changes in this PR:

* Support for Transactional HTTP Endpoint, new in Neo4j 2.0 - see #15 
* Support for batched Cypher queries - see #2 
* Support for unmarshalling results of Cypher queries into structs.
* Changes `Database.Cypher()` to accept a single `*CypherQuery` argument instead of query, params, and result.