Click parent anchors on enter and space

[kennethklee]

Not sure of the proper protocol or if this is a sufficient PR. Hopefully this helps though.

Changes:

• Add link attribute to paper-button to stay consistent with paper-tabs.
• Add handler for tap to trigger the parent anchor element if it exists.

Fixes #147.

[googlebot]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

:memo: Please visit https://cla.developers.google.com/ to sign.

Once you've signed, please reply here (e.g. I signed it!) and we'll verify. Thanks.

---

• If you've already signed a CLA, it's possible we don't have your GitHub username or you're using a different email address. Check your existing CLA data and verify that your email is set on your git commits.
• If you signed the CLA as a corporation, please let us know the company's name.

[kennethklee]

I signed it!

[googlebot]

CLAs look good, thanks!

[kennethklee]

Lemme know if this is good, and I'll do the same to paper-icon-button.

[kennethklee]

seems to work without this.