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 :-) !
still the same :-) In the datasheet for the device they define the minimum timings you have to comply to. And by looking at the output of your logic analyzer, you can measure the timings that are actually put on the pins. If these timings are sufficiently long without calling any `hwtimer_spin` or `hwtimer_wait`, then you don't need to call them, and I would strongly vote against using them if not strictly necessary for efficiency reasons...