Svg header

Added a .pxd header for the SVG library, so that other files can cimport Svg. 
@sbrother, developers from Google contribute frequently to the Linux kernel, yet the company name is not in the [credits](https://www.kernel.org/pub/linux/kernel/CREDITS), nor are there any other companies listed.
I believe it is best if Kivy's contributor list remains to be a list of humans, as it is common practice.