[DOM-Level-3-Events-key] For user-agents on Windows OS, map the "windows key" to Meta (instead of "OS") (#9)

For KeyboardEvent.key values on Windows OS, use "Meta" (instead of "OS"). This is for 1) better interop with Mac, 2) enabling general use of the ```meta``` attribute on KeyboardEvent.

See discussion at: https://github.com/w3c/uievents/issues/28#issuecomment-151028832

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/DOM-Level-3-Events-key/issues/9