Add the AODVv2 Routing Protocol

This PR depends on #1766.

It contains a minimal implementation of the AODVv2 routing protocol.
*Not* implemented are:

	- AckReqs
	- alternate metrics
	- multiple interfaces
	- clients and Client Networks
	- buffering
	- all addresses, TLVs, and features that are marked as optional

An example application can be found at https://github.com/Lotterleben/RIOT-AODVv2/tree/master/aodvv2_demo.

The implementation relies heavily on a functioning Neighbor Discovery Protocol.
It might be necessary to fill the neighbor cache manually with the current state
of RIOTs NDP implementation.

The value of AODVV2_MAX_UNREACHABLE_NODES has been chosen arbitrarily and will be subject to
future improvement.

Please note that based on my experience, with the default transceiver
buffer size (3) of the native port, about 2/3 of the route discoveries
will fail. This has been addressed in issue #1747. It is advised to increase
the transceiver buffer size when using AODVv2 as a routing protocol.
I think using `size_t` for the len parameter would be saner. Or is there any case where one would need negative values here?