Address mvano's comments on binary PushMessageData

PushEventInit and it's data member are now optional again:
https://github.com/w3c/push-api/pull/88#discussion_r20867428

Tweaked IDL spec deeplink:
https://github.com/w3c/push-api/pull/88#discussion_r20864592

Tweaked respec linking:
https://github.com/w3c/push-api/pull/88#discussion_r20864731

Clarified JSON.parse exception re-throw:
https://github.com/w3c/push-api/pull/88#discussion_r20864930

Addressed <li><p> nit:
https://github.com/w3c/push-api/pull/88#discussion_r20865058

Defined what happens if data attribute is not set:
https://github.com/w3c/push-api/pull/88#discussion_r20865550