Improve transformer

[jamesfer]

Makes some long needed updates to the transformer setup:

• [x] Change transformer class into a function to make it more flexible.
• [ ] Add the transformer as a parameter on the connection to allow customisation.
• [ ] Add the option to make the transformer more transparent (return all Neo4j objects unmodified) - fixes #122
• [ ] Change the return type of query methods to match the return type of the transformer - should fix #119