radio: Remove old radio events documentation
nrf51822/radio: Set inter frame space to 150 us
nrf51822/radio: Add reference for radio mode
nrf51822/radio: Create define to base radio shorts
nrf51822/radio: Remove other shorts than base ones when stop the radio

This prevents triggering unwanted shorts right after stop the radio.
nrf51822/radio: Fix NULL radio driver bug

The radio should operate normally without a driver.
radio: Add radio_prepare() API
nrf51822/radio: Implement radio_prepare() API
radio: Remove ch/aa/crcinit params from radio_send/recv() APIs
examples: Replace radio_reply() by radio_send()
radio: Remove radio_reply() API
radio: Remove len parameter from radio_send() API
radio: Add flags parameter to radio_send() API
nrf51822/radio: Clear flags when stop the radio
radio: Add radio_set_out_buffer() API
radio: Add flags parameter to radio_recv() API
radio: Modify radio callbacks to return boolean values

Radio callbacks now returns false when they want to stop radio
activity. If they return true, the radio operation isn't modified.
examples: Add radio-active-observer example