added board airfy beacon

wiki page: https://github.com/RIOT-OS/RIOT/wiki/Board%3A-Airfy-Beacon

the following PRs increase the usability of the nrf51 platform in general:
- for interrupt driven UART: https://github.com/RIOT-OS/RIOT/pull/1555
- for working timers: https://github.com/RIOT-OS/RIOT/pull/1824 and https://github.com/RIOT-OS/RIOT/pull/1827
- for the radio: https://github.com/RIOT-OS/RIOT/pull/1537

wishlist:
- proper *power modes* are missing too
I don't quite understand why you add these additional GPIOs, you don't even define them in the `periph_conf.h`...