Svg header

Added a .pxd header for the SVG library, so that other files can cimport Svg. 
So, Google asserts copyright over the open source work that I create during my employment there, unless I go through a lengthy exception process (which is targeted more toward employees who want to own their own work, not open source it). So basically in the absence of that agreement, I would maintain the copyright to the code added to Kivy, but I would be releasing it under the terms of the MIT license. Under the agreement, Google owns the copyright to the code I add to Kivy, and they release it under the MIT license.

Maybe "Contributors" isn't the right place to put them, though, since they are not a person and I am the one who wrote the code. Is there a separate "Copyright" section, either in the source or on the Kivy website somewhere?