RTC shell command update

* rewrite rtc command for periph/rtc
* adopt default application

**Based on** ~~https://github.com/RIOT-OS/RIOT/pull/2060 and~~ https://github.com/RIOT-OS/RIOT/pull/1922

Update;
Removed `strptime` for now
Not sure, the interface does not specify what kind of error triggers this. We could indicate the command which failed.