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 :-) !
No it's not really necessary but i found it safer to have it in, cause the time between a CS low to high transition (end of spi command) and a CS high to low transition (begin next spi command) is really short without delay. I will set it to zero but let the function call in for the first. D'accord?