sys: net: Initial import of a basic MAC protocol

This is basically an adapter translating `netapi` IPC calls into `netdev` function calls.

Depends on ~~#1448~~ (merged) and ~~#2027~~ (merged)
```
[martine@beutlin RIOT]<3 git checkout master 
Already on 'master'
Your branch is up-to-date with 'upstream/master'.
[martine@beutlin RIOT]<3 git pull
Already up-to-date.
[martine@beutlin RIOT]<3 git grep "while *(0)" | wc -l
56
```