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~~ (merged)
An application should not have to bother with the nrf24 directly, unless it really wants to.
In the first case, the netdev API will dictate the implementation,
in the second case the user wants to deal with this stuff.
=> Don't add convenience as part of this PR.