http://docs.neo4j.org/chunked/stable/rest-api-transactional.html
Looking at that, you can see the body of your POST request isn't what the server is expecting, i.e. you should be sending an entire JSON document, and not a k/v pair w/ "statements" as a key and your JSON Cypher query as the value. Remember you're sending JSON here, and not a URLEncoded body.
Also, it looks like you're setting the "Accept" and "Content-Type" k/v pairs as part of the POST body when they should, in fact, be part of the headers.
Also also, consider using the Cypher endpoint: http://docs.neo4j.org/chunked/stable/rest-api-cypher.html
HTH