Enhancement - Optional iron-page-behavior for childs with select / deselect lifecycle callbacks.

[bwh-rl]

Main reason for this would be the view switching like shown in the starter-kit.
e.g. If i have an Video on an view element, you want it to stop it on deselect.
Or reloading ajax requests on select, or resetting form data to blank, ...

On select / deselect the iron-pages element would have to check if the selected / deselected element got the iron-page-behavior, and call the function. The usage without that feature would not change at all.

[bwh-rl]

Maybe an child behavior is not needed at all.
Just check if the function exist and call it.

[bwh-rl]

Moved PolymerElements/iron-selector#140