sys/net/network_layer added a core implementation of a FIB 

This PR introduces a _Forwarding Information Base_ (FIB) implementation.
It will replace the `ip_get_next_hop()` call on registered functions from routing protocols (RPs), e.g. [here](https://github.com/RIOT-OS/RIOT/blob/master/sys/net/network_layer/sixlowpan/ip.c#L114).
* It provides management functions that must be used by RPs to add, delete and update next hop entries.
* It signals reactive routing protocols whenever the FIB does not provide a next hop towards a destination to start a route discovery.
* Underneath it provides a `universal address container` that allows to use one added entry multiple times.
* The FIB is completely event triggered (no threads used)
* All access is mutually excluded to allow being operated from asynchronous sources.
done.