MessageChannel based filesystem

[humphd]

For our Brackets work, I wrote a remote proxy for Filer that runs over a MessageChannel between two windows. It works really well, and lets you do cross-origin filesystems easily. Firefox doesn't currently have MessageChannel on by default, but it's implemented, and I've also got a working shim that uses postMessage.

Here's the PR in case someone wants to do this work in Filer itself some time.

https://github.com/humphd/brackets/pull/329