Add xmlns to defs sprite

From http://css-tricks.com/svg-sprites-use-better-icon-fonts/

In testing, Jonathan Neal discovered you need to have the xmlns attribute on the <svg> for it to work: <svg xmlns="http://www.w3.org/2000/svg">

Confirmed this fixed my issue when running this locally
Updated tests
Merge pull request #14 from thomaswelton/patch-1

Add xmlns to defs sprite