Errors in ssh_tty.py's read_prompt method

[jeffbrl]

I believe the code in ssh_tty.py's read_prompt method has several errors. What's puzzling me though is that no one else seems to have encountered an exception when the current code is executed. I see examples on the web with using netconify with ssh.

   def read_prompt(self):
        got = []
        mark_start = datetime.now()
        mark_end = mark_start + timedelta(seconds=15)

        while datetime.now() < mark_end:
            delta = time.time() - start
            time.sleep(0.1)
            rd, wr, err = select([self._chan], [], [], self.SELECT_WAIT)
            sleep(0.05)
<snip>

1. The start variable should be mark_start
2. You can't subtract a datetime object from a float
3. sleep is imported from time currently. The import should be changed to 'from time import sleep, time. Once that change is made, both calls to sleep should be simply sleep.

I'll submit a PR for the fixes.