update changelog

add core-scroll-threshold example

get grouped flickr demo working

initial port of core-list and grid demo

ported core_ajax with tests/examples

clean lib/src dir, run bower update, and run custom element apigen

BUG= https://github.com/dart-lang/core-elements/issues/161
R=sigmund@google.com

Review URL: https://codereview.chromium.org//770003004