Svg header

Added a .pxd header for the SVG library, so that other files can cimport Svg. 
That's convincing enough for me. FWIW, I did it this way because I'd read this exchange over at the Snap Framework: https://github.com/snapframework/snap-server/commit/5e4850d4cb74c75e706df509b90befc95263746a

But that was on a copyright page that already had organizations, not humans on it. I think as long as there's a record (here) saying that Google owns the copyright on my commits since June 1, 2014 then this ought to be fine.

Thanks for reviewing and merging!