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 and I have been thinking about that. Maybe its a better idea to have an interface function in any mode, but if such a packet (and this should not be the case due to the source_routing indicator) arrives in storing mode, the appropriate function does nothing but return. With this homogeneous interface for all modes, we can than switch in the end anytime. 

Also with this I can extract this part into the non-storing mode.