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
For naming conventions I would prefer a different prefix (something like `_native_` for internal variables.