PlatformIO-based manifest file

* Your library in Web: http://platformio.ikravets.com/#!/lib/show/Arduino-DS3232RTC
* PlatformIO Library Manager: http://docs.platformio.ikravets.com/en/latest/librarymanager/index.html
* Arduino IDE: [Integration of PlatformIO library manager to Arduino IDE](http://www.ikravets.com/computer-life/platformio/2014/10/07/integration-of-platformio-library-manager-to-arduino-and-energia-ides)