Support embedded SVG graphics and custom vector Graphics2D drawing

I've added two commands ```:graphics``` and ```:svg``` which allow fancy-dan vector graphics to be embedded in PDF pages, either above the document page or under it. Have a look at the generated example.pdf here: https://github.com/rm-hull/clj-pdf/blob/embedded-svg/example.pdf?raw=true.

I've updated the README.md with the command usage and the example near the end. Hopefully it is fairly straightfoward to follow the changes I made.

I think it would be possible to get *JFreeChart* rendering to pass through this instead of (or as well as) using the ```:image``` tag so we could get nice crisp vector charts. However, it writes directly to the page rather than respecting the document flow. I'll perhaps have a look at this next, and submit it as another pull request.

I also bumped up the *iText* and *JFreeChart* versions to the next minor increment along, and got rid of surplus EOL whitespace - I guess you can choose to ignore that commit if necessary.