Transfer files between client and worker contexts as transferrable object

[shichongrui]

Until data channels are implemented in the worker, phone-a-friend must do all webRTC related actions in the client. Then messages are passed back and forth between the worker and the client to transfer files and request files from peers. It is currently using MessageChannels which has a bug where transferrable objects come out as null on the second port. https://code.google.com/p/chromium/issues/detail?id=334408 This may only be possible when a peer receives a file response and then passes that object into the worker. We don't want the worker to loose the file when it passes the context to the client.