updating codelab with app changes, <polymer-element>, etc.
Merge branch 'master' of git://github.com/ebidel/polymer-todomvc-codelab

Conflicts:
	step1_soln/elements/td-item.html
	step1_soln/elements/td-todos.html
	step2_soln/elements/td-todos.html
	step3/elements/td-item.html
	step3/elements/td-model.html
	step3/elements/td-todos.html
	step3_soln/elements/td-model.html
	step3_soln/elements/td-todos.html
	step4/elements/td-item.html
	step4/elements/td-model.html
	step4/elements/td-todos.html
	step4_soln/elements/td-item.html
	step4_soln/elements/td-model.html
	step4_soln/elements/td-todos.html
	step5/elements/td-item.html
	step5/elements/td-model.html
	step5/elements/td-todos.html
	step5_soln/elements/td-item.html
	step5_soln/elements/td-model.html
	step5_soln/elements/td-todos.html
	step6/elements/td-item.html
	step6/elements/td-model.html
	step6/elements/td-todos.html
	step6_soln/elements/td-item.html
	step6_soln/elements/td-model.html
	step6_soln/elements/td-todos.html