Fix to issue #7. Only attempt to add the "transform" method to d3.svg if it is present.  Export the "transform" method via module.exports if module.exports is defined, otherwise add it as d3Transform to the global namespace.
Move the node.js usage instructions in README.md

Shifted and modified the wording of the node.js usage instructions in README.md
Merge pull request #8 from davidbkemp/master

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