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
s/elment/element/