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 (@hauke is working on a new implementation)

wishlist:
- proper *power modes* are missing too (probably simple)