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)
Of course this PR is a predecessor to the (hopefully soon) coming PR including the netdev api (most things are already implemented for this). So in here, I just wanted to show/test the low end side of the driver. And with commit 26ee930b3bb228e1b9fbd6033ac48d0c3f2c31b1 I changed the receive procedure to the idea of @haukepetersen. You think it's ok this way for this PR or what exactly needs to be changed?