Provide a way to support animations associated with data changes which mutate dom

[sorvell]

Here's a simple example using template if:

http://jsbin.com/qilixe/2/edit

Additional discussion: https://groups.google.com/forum/?utm_medium=email&utm_source=footer#!msg/polymer-dev/Sp0rJmitpNQ/RIX1xpOKOGAJ

[tjsavage]

Closing this issue due to age and the release of version 1 of Polymer - please feel free to re-open if this is incorrect.