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 :-) !
Are you talking especially about line 44? Because I put this delay at the end of each function that uses spi transfers... So, what I wanted was less a delay after disabling an spi-device but more a delay before each new spi-usage. Maybe (even if I set the hwtimer_wait value to 0 ticks) it would be more clear if this delay would be at the beginning of each function that uses spi. Nevertheless, this is just for additional safety/flexibility, e.g. when a higher datarate is set up. But it's still not necessary. So what do you think now?