[Alarm]: Implementation of Tizen alarm API.

This commit implements the Alarm API:
  https://developer.tizen.org/dev-guide/2.2.0/org.tizen.web.device.apireference/tizen/alarm.html

There is a known issue about repeat alarm: if the working system time zone is set to Daylight saving time like PDT or EDT, the repeat alarm finds 3600 seconds gap on the initial trigger time.

BUG=XWALK-428
The 64 indicates the maximum size of the internal buffer to hold the returned value.
This is declared in the base class:
std::string Extension::GetRuntimeVariable(const char* var_name, unsigned len);
I feel the GetRuntimeVariable need to be improved to be:
  std::string Extension::GetRuntimeVariable(const char* var_name);
The client does not need to pass the length. This function should take care the internal buffer.