validate input on blur

[notwaldorf]

• validates on blur, not on attached
• changed the value observer so that we can get the previous value, and prevent auto-validating on ready
• fixed the broken caret-preserving test (which wasn't actually doing anything)

This matches the behaviour in these other PRs:

• [ ] https://github.com/PolymerElements/paper-input/pull/148
• [ ] https://github.com/PolymerElements/gold-email-input/pull/29
• [ ] https://github.com/PolymerElements/gold-cc-input/pull/29
• [ ] https://github.com/PolymerElements/gold-cc-cvc-input/pull/23

[cdata]

I think most of my feedback on this one carries over to the other three.

[notwaldorf]

@cdata I have fixed all the things! And made them better, I think. PTAL?

[cdata]

LGTM :rocket: