Update polymer tutorial; make directory paths match new sample structure

The polymer-relevant text changes are in this page:
https://polymer-dot-dart-lang.appspot.com/docs/tutorials/polymer-intro/

Search the patch set list of files for "markdown" to find other text changes
and "images" to find image changes.

Please also look at the code changes, which are here:
https://github.com/dart-lang/dart-tutorials-samples/pull/15

R=sigmund@google.com

Review URL: https://chromiumcodereview.appspot.com/275613002