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.
Right - in future this should be improved. I think there is no real good decision in this situation. If we have different modules, every mode and submode should have one - that means fragmentation. However for a user it is not a good idea to change it by looking into the config every time - the Makefile would be a proper place. Any advise what would be best?