Add tests

For now, these cover only the library and don't attmept 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.