Improve transformer

[jamesfer]

Makes some long needed updates to the transformer setup:

• 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 Transformation of Temporal type #122
• Change the return type of query methods to match the return type of the transformer - should fix Cannot specify query return type with strict keys #119