Implemented a new property for writable file streams that keeps track
of the bytes written (not queued). This helps when you are piping
another stream to a file, and would like to know how big the file is
without having to issue another stat call.
closes#930
Allows for more fine graining, especially finding out about an individual
chunk of data being flushed in a write stream rather than the whole queue.
This commit also fixes a bug causing forceClose to fail on a readStream that
did not finish opening yet.