drivers: Introduce a general interface for network device drivers

This is thought to merge the concepts of @rousselk's #925 and my #1448. It also addresses https://github.com/RIOT-OS/RIOT/pull/1448#issuecomment-50395820

Things to do:

- [x] make `ieee802154_radio_driver_t` subclass of `net_dev_driver_t`
- [x] rename to naming scheme where renaming is due
- [x] adapt already implemented drivers
- [x] add native support
- [x] provide tests
I guess I could check `ptr` instead, but the whole thinking about making the check there and not further down is that it is just my style to make all variable declarations first and then come with code ;-)