[IndexedDB] Add scope attribute to IDBTransaction (#18)

I was thinking `scope` but FF seems to have `objectStoreNames` so we can just standardize that.

http://lxr.mozilla.org/mozilla-central/source/dom/webidl/IDBTransaction.webidl

TODO: test what this returns for versionchange transactions. (empty? all current?)


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https://github.com/w3c/IndexedDB/issues/18