Introduction of RPL non-storing mode

This is the first commit for discussion. It works - but e.g. the checksum in UDP-packages is wrong because the length is zero (which it is not btw). In addition the design decisions concerning messages sending/relaying may be discuss-able - but I think they are as noninvasive as the network stack would allow them to be.

At last - the proposed fixes concerning storing/non-storing mode files with the same declaration-base in one directory from @Kijewski does not work. (Both - excluding files or moving them to a sub-directory) But I will see more into it. Maybe someone can help.
First of all you're right, that things like this are done in (too) many places in RIOT currently. However, I'd like to minimize memory usage in our network stack over time and try to avoid adding more operations like this.

In the current implementation the initial buffer should be always big enough to hold the full IPv6 packet if the packet does not exceed the MTU IIRC, so there should be a way to insert the SRH in-place by shifting the following content to the right. However, since @authmillenon is heavily refactoring buffer usage in the IP stack, maybe she has a better recommendation how to handle this for now.