cpu/nrf51822: added support for the NRF51 radio

rebased on #2005

Finally support for the NRF51 radio that is part of the NRF51822 SoC. I took the design decision to name the radio simply `NRF51` and to include it as it's own module in the cpu folder instead of an peripheral driver. This makes the integration in existing network modules (i.e. transceiver) easier...

The driver itself is quite rudimentary for now, it basically shuffles bytes of data over a wireless link... The next steps are
- use some kind of frame format (maybe `ieee802154_frame_t`) for packaging the data to be send
- adapt the `netdev` interface
- configure alternative device configurations for using the radio in BLE environments