Let the framer functions use ints instead of uint8_t to make them more generic
A number of updates to nullrdc

* add a few rimestats to keep track of sent and received acks
* made a number of configuration options possible to override (ack timing)
* added the logic for sending 802.15.4 link layer ack packets, despite not being able to guarentee the 802.15.4 MAC timing
* increased the number of sequence numbers to keep track of for duplicate filtering
Send announcements only if the node is configured to be a router
New mechanism for computing the 6lowpan header length dynamically, depending on how large header the framer creates. Also check the incoming packet for buffer overflow.
Merge pull request #174 from adamdunkels/feature-net-improvements

Improvements to the core/net/ code