Add tests

For now, these cover only the library and don't attempt to do any integration tests in the browser. It's probably worth investigating using something along the lines of PhantomJS in the future.

I've created a test assets by saving the SVG output from each generator. The tests run by comparing the XML generated by the library against the saved SVG file. A simple string diff seemed too unreliable, but I've encountered no issues with an XML diff so far. That said, better ideas are welcome.