Changed fill property to color attribute

While following the tutorial, I found the fill property doesn't toggle the color correctly in chrome,checking the demo, this works using the color property instead.
Merge pull request #604 from thegecko/patch-1

Changed fill property to color attribute
Formatting fixes.
Merge branch 'master' of https://github.com/Polymer/docs into tutorial-update-0.3.5

Conflicts:
	docs/start/tutorial/step-4.md