sys: net: Initial import of a simple data-forwarding MAC protocol (nomac)

This is basically an adapter translating `netapi` IPC calls into `netdev` function calls.

Depends on ~~#1448~~ (merged) and ~~#2027~~ (merged)
> But this shouldn't work because at this point thread_getpid() does not return the pid of nomac's thread! _nomac_recv_cb is processed in a different process (the one where the receive of the device is triggered) and the equality check _nomac_registry[i].registrar_pid == current_pid fails.

If this happens the driver is implemented wrong. The normal sequence of events for netdev is (as specified [here](https://github.com/RIOT-OS/RIOT/wiki/Model-for-the-network-stack#receive-packet-from-hardware)):
1. Receive triggered
2. Driver sends `NETDEV_MSG_EVENT_TYPE` message with a driver-locale receive event identifying value `v` to `nomac` in this case
3. MAC receives `NETDEV_MSG_EVENT_TYPE` and calls `dev->driver->event(v)`
4. driver copies data into packet buffer
6. driver calls `_nomac_recv_cb`