title: neo4j - py2neo - Unable to place parameters into relationship on creation
body: <p>If I run my python script over neo4j database, works ok:</p>
  
  <pre>
  from py2neo import Node, Relationship, Graph, cypher, authenticate
  
  # set up authentication parameters
  authenticate("localhost:7474", "user", "password")
  
  # connect to authenticated graph database
  sgraph = Graph("http://localhost:7474/db/data/")
  
      a=raw_input("Input Name A: ")
      b=raw_input("Input Name B: ")
      graph = Graph()
      tx = graph.cypher.begin()
      tx.append("MATCH (c:Person {name:{a}}), (d:Person {name:{b}}) CREATE (c)-[:KNOWS}]->(d)", a=a, b=b)
      tx.commit()
  </pre>
  
  <p>but, when I tried to add parameters to relationship...:</p>
  
  <pre><code>a=raw_input("Input Name A: ")
  b=raw_input("Input Name B: ")
  z=raw_input("Input parameter of relationship z: ")
  x=raw_input("Input parameter of relationship x: ")
  graph = Graph()
  tx = graph.cypher.begin()
  tx.append("MATCH (c:Person {name:{a}}), (d:Person {name:{b}}) CREATE (c)-[:KNOWS{labelz:{z},labelx:{x}}]-&gt;(d)", a=a, b=b)
  tx.commit()
  </code></pre>
  
  <p>I got this:</p>
  
  <pre>
      tx.commit()
   File "/usr/local/lib/python2.7/dist-packages/py2neo/cypher/core.py", line 333, in commit
      return self.post(self.__commit or self.__begin_commit)
   File "/usr/local/lib/python2.7/dist-packages/py2neo/cypher/core.py", line 288, in post raise self.error_class.hydrate(error)
  
  py2neo.cypher.error.statement.ParameterMissing: Expected a parameter named z
  </pre>
  
  <p>How do I have to place those variables to avoid that errors?
  Thanks in advance.</p>