Enable clients to use CSS animations.

[cbelden]

This is an idea that solves an issue we were experiencing with the paper-dialog. We found that modifying the existing animations (ie "scale-up-animation") would require either implementing our own web animation or providing some sort of global animation configuration.

Using CSS animations is an easy way for us to apply an animation to all of our paper-dialog instances, however, there was no decent way to target dialogs that are opening/closing.

This PR adds classes when the dialog is opened (or closed). This PR also resolves issue #119 (or at least provides a workaround).

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[cbelden]

I signed it!