Add backend APIs for issuing U2F requests and processing U2F responses.

[xiaoliu1988]

Hi @leshi, please review this change!

Endpoint APIs include:

1. return all registered security keys;
2. issue a registration request;
3. process a registration reponse;
4. issue a signature request;
5. process a signature response.

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[xiaoliu1988]

I signed it!

On Tue, Feb 28, 2017 at 2:45 PM, googlebot notifications@github.com wrote:

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).

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[googlebot]

CLAs look good, thanks!

[xiaoliu1988]

This one will be closed once #152 is reviewed.