Rename web-animation.js to web-animations.js

Also rename web-animation-test.html to web-animations-test.html and
web-animation-visualization.js to web-animations-visualization.js.

This fixes https://github.com/web-animations/web-animations-js/issues/113
Merge pull request #119 from steveblock/rename

Rename web-animation.js to web-animations.js