added keyboard navigation to link tabs

[jannakha]

added core-a11y-keys element to paper-tab for keyboard navigation

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[robdodson]

@morethanreal would it be possible to merge this in + mixing in Polymer.CoreFocusable?

[robdodson]

sorry, just realized I should have pinged @frankiefu

[jannakha]

I signed CLA, it still shows as 'no'

[googlebot]

CLAs look good, thanks!

[robdodson]

@jannakha do you know if this works if there's an anchor inside of the paper-tab? will the listener for enter intercept it and prevent the anchor from being clicked?