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
Disable generation of html previews
Use relative units for sprite placement
Add slider to demo page to adjust font size
Update tests
Add new config var generatePreview to togle preview generation
Update test for HTML preview correctness
Add test to verify preview is not generated
Merge branch 'refs/heads/patch-1'
Merge branch 'refs/heads/features/no-preview'
Merge branch 'refs/heads/relative-sizing'