Created new Conn.Flush() public method so the fd pipeline can be drained arbitrarily...

Created new Conn.Flush() public method so the fd pipeline can be drained arbitrarily by the user.
Commented both flush methods so people know what they are looking at.

This is a necessary fix for streaming and long polling HTTP services.
Fixes #93.

R=r, rsc, david.titarenco
https://golang.org/cl/154099