Move away from pytz to do timezone math.

The calculation for the time until the next standup was an hour off.
This is because pytz is deeply broken:

  >>> import datetime, pytz
  >>> int(datetime.datetime.now().strftime("%s")) - int(datetime.datetime.now().replace(tzinfo=pytz.timezone("US/Pacific")).strftime("%s"))
  -3600
  >>> datetime.datetime(2013, 8, 27, 16, 46, 8, 469362).timetuple()
  time.struct_time(tm_year=2013, tm_mon=8, tm_mday=27, tm_hour=16, tm_min=46, tm_sec=8, tm_wday=1, tm_yday=239, tm_isdst=-1)
  >>> datetime.datetime(2013, 8, 27, 16, 46, 8, 469362,
  >>> pytz.timezone("US/Pacific")).timetuple()
  time.struct_time(tm_year=2013, tm_mon=8, tm_mday=27, tm_hour=16, tm_min=46, tm_sec=8, tm_wday=1, tm_yday=239, tm_isdst=0)

Switch to the 'tzlocal' functionality within dateutil, which seems to
work as you'd expect.