drivers/sys : Initial import of high-level driver for reader/writers

After some discussion with @haukepetersen a few weeks ago (and @thomaseichinger and @OlegHahm yesterday), I was thinking about a unified interface for radio chips, files, sockets, network stacks, serial interfaces, etc. a lot and now I finally got some time to formulate my idea (in the spirit of Hauke's #612, #613, #614, #615, and #616). It could completely replace the transceiver module, the (not merged from #460 yet) net_if module, and would greatly simplify POSIX wrappers, since this basically equalizes PIDs and file handlers.
This does not make #460 obsolete for now. In fact: #460 would greatly smooth the transition from the current state to the state of this proposal.