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.
I think thats the main flaw of the RPL RFC. The root sure has to be more powerful (as advised several times in the document) but at other stages it is said, that a node should be able to change its role to be as adaptive as possible. Of course these two things don´t coincide. 

However I see your point and this is fine by me. I will introduce that with the coming commit update.