implement basic nRF24L01+ support
[nrf] missing include
[nrf] fix data rate enum
[nrf] add power saving mode

remove public read/write register functions
[nrf] set the correct ARD value for 250kbps data rate
[nrf] future api
[nrf] dynamic payload (wip)
[nrf] fix crc bug

add vim modelines
[nrf] fix crc bug introduced by 4cf482c0ce, optimize dyn payload setup
[nrf] add crc length config call

remove warnings and some comments
[nrf] set RT params and send a ping at the beginning
Merge pull request #77 from tralamazza/nrf_lib

implement basic nRF24L01+ support