feat(iron-autogrow-textarea.html): add debounce function

[felixzapata]

This fixes #116, adding a debounce function to avoid the delay to the user.

[notwaldorf]

I don't think this is good, since it basically delays everything by half a second. In particular, this also breaks all the tests, which means it breaks the tests of anybody who's ever written a test for using this element in their app.