Fix to issue #7. Only attempt to add the "transform" method to d3.svg if d3.svg is present.

Fix to issue #7. Only attempt to add the "transform" method to d3.svg if d3.svg is present. Export the "transform" method via module.exports if module.exports is defined, otherwise add it as d3Transform to the global namespace.

Let me know if you would like me to add some examples and/or more documentation.