[Twisted-Python] twisted.lumberjack clobbers existing logs

[Andrew Bennetts]

Hi everyone...

I like the twisted.lumberjack module that was recently added, but I think it
has a small buglet.  It checks the size of the existing log file, but then
opens it in mode "w", truncating it.

Thus, I suggest the last line of LogFile._openFile should change from:
        self._file = open(self.path, "w")
to:
        self._file = open(self.path, "a")

-Andrew.