radio: Add Radio APIs
nrf51822: Implement radio_init()
nrf51822: Implement radio_register_handler()
nrf51822: Implement radio_recv()
[NOT TESTED] nrf51822: Implement radio_stop()
[WIP: missing timers] examples: Add radio-observer example

This patch adds an example that implements the GAP observer role using
radio and timer APIs directly.
[NOT TESTED] nrf51822: Implement radio_send()