Use createElementNS to allow running in an SVG document

Fixes https://github.com/web-animations/web-animations-js/issues/12
Merge pull request #265 from dstockwell/create-element

Use createElementNS to allow running in an SVG document
Merge remote-tracking branch 'web-animations-next/master' into HEAD
Add build artifacts from 8032adef386638189cc18f3468f39f6ddef8ffe5