drivers/nrf24l01+ : added low-level functions for nrf24l01+ transceiver

This PR contains multiple low-level functions for the nrf24l01+ driver. Maybe some of them seem useless here but these are interesting  when it comes to bind the driver to the general network driver interface, which will be part of another PR soon. Furthermore there are some functions on which I'm not sure if they are necessary. Feel free to discuss :-) !
In the datasheet on page 52-53 you find T_cwh which defines the "dead time" between a CS high and CS low again. It is meant to be 50 ns. Without any delay inside the lowlevel functions, this gets critical sometimes. Now I put the smallest possible delay at this position (`hwtimer_spin(1)`) which makes approx. 1 us delay (this is a really high value :/ ). However, it is important for a safe communication.