Use readline on the input fd instead of iterating

[sigmaris]

In Python 2.x, using 'for line in self.fd' causes messages to be delayed and arrive in bursts due to internal read-ahead buffering, see http://bugs.python.org/issue3907 and http://stackoverflow.com/questions/2804543/read-subprocess-stdout-line-by-line Using readline() explicitly avoids this problem as it doesn't read ahead.