core: priority_queue: simplify inheritance
vtimer: no needless type casting

Compare https://github.com/RIOT-OS/RIOT/pull/1639#issuecomment-54713048
and following discussion.
Merge pull request #1644 from Kijewski/vtimer-less-casting

vtimer: no needless type casting
Merge pull request #1639 from authmillenon/simplify_queue_inheritance

core: priority_queue: simplify inheritance
net: Initial import of a global packet buffer
Merge branch 'packetbuf2' into uart_net
Initial import of SLIP networking module