Assorted tutorial fixes. Requires updated version of tutorial repo.
Merge branch 'master' of https://github.com/Polymer/docs into tutorial-updates

Conflicts:
	docs/start/tutorial/intro.md
Fix merge conflict
Update screenshot style to match intro
Update sample files, too
Fixed link style for element links, added a couple more links.
Merge pull request #497 from arthurevans/tutorial-updates

Tutorial updates