Use PathObserver.setValue for DOM->model

Use PathObserver.setValue for DOM->model

R=arv
BUG=

Committed: https://github.com/Polymer/NodeBind/commit/1e78b2c

[rafaelw, 2013-08-29 17:58:11]

Note that right now, this means that if the model provided is a
CompoundPathObserver, pushing the value into the observer will have no effect
(the value will just get dropped).

[rafaelw, 2013-08-29 17:58:40]

Committed patchset #1 manually as r1e78b2c (presubmit successful).

[arv, 2013-08-29 18:05:20]

LGTM