bugs in get_idle_periods, with suggested fixes #122

Sorry for not sending a PR, not working on a computer set up for git etc.

1. _e3dc.py line 462:
   if idlePeriodsRaw[0] != RscpTag.EMS_GET_IDLE_PERIODS:
   Looks like the first value in idlePeriodsRaw can be a string, not just the enum. This breaks the code even when the device replies correctly.
   This works and should catch all variations:
   if getRscpTag(idlePeriodsRaw[0]) != RscpTag.EMS_GET_IDLE_PERIODS:

2. _e3dc.py lines 466-467:
   "idleCharge": [] * 7, "idleDischarge": [] * 7,
   This can not work, []*7 is still just []. You need something in there to create a list of that size, e.g.: [None] * 7
   Without this the assignments on lines 489/491 always throw exceptions.

Thanks!