Enhancement - Add select / deselect lifecycle callbacks. #140

[bwh-rl]

Main reason for this would be the iron-pages 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, ...

[bwh-rl]

I think this would also apply to PolymerElements/iron-pages#47