BroadcastChannel polyfill

This is a polyfill for BroadcastChannel.

It is just a published copy of Joshua Bell's gist (since broadcast-channel polyfill won't work for our project).

It requires Message Channel support, so should work in:

• Chrome 4+
• Safari 5+
• Opera 11.5+

Does not work in:

• Firefox 37- (neither BroadcastChannel nor MessageChannel)
• IE (??)

Caveats

• The real API should let you transmit anything which can copied by the structured cloning algorithm. This polyfill only copies things using JSON.stringify()/JSON.parse() so it is much more limited.
• This polyfill uses DOM Storage (localStorage) and storage events. DOM Storage is a synchronous API and so may cause performance issues in pages. In addition, it is not exposed to Workers. Therefore, the polyfill will not function in Workers.
• Unique storage keys are used for each message, and are cleaned up a few hundred milliseconds after transmission. This is a total hack and may result in the messages failing to be received (if the write and delete are coalesced) or persisting (if the cleanup is prevented by page close).