Fix coping and moving files around using the file tree (#568)

* Implement write/read buffers in electron fill

This makes cutting and copy files from the file tree work.

* Implement fs.createReadStream

This is used by the file tree to copy files.

* Allow passing proxies back from client to server

This makes things like piping streams possible.

* Synchronously bind to proxy events

This eliminates any chance whatsoever of missing events due to binding
too late.

* Make it possible to bind some events on demand

* Add some protocol documentation