move accessor binding into polymer

Note that this patch just moves code from observe-js into polymer (with slight tweaks).

Follow-on work pending:
-dont force creation of an array literal for registerObservers
-cleanUp the close*Observer mess
-add asserts to unbind tests that zero observers are active
-allow computed properties to be bindable

sorvell
BUG=
R=sorvell@google.com

Review URL: https://codereview.appspot.com/104910045