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 :-) !

Depends on https://github.com/RIOT-OS/RIOT/pull/1584
I would say it can be deleted. So there are some functions that were in your first suggestion of this driver, on which I'm not shure if they are really needed or what you would have seen inside. These are for example: `nrf24l01p_init_auto`, `nrf24l01p_register`, `exti_cb`...