6lowpan fragmentation fix

We found a pretty hard-to-track bug in the 6lowpan fragmentation code that caused almost total network breakdown when there were many fragmented packets around (more explanation in https://github.com/adamdunkels/contiki-fork/commit/241a390b1dfa76faa226c610bb1689476cc08b7e). This pull request fixes the bug and adds a set of regression tests that checks for regression for this particular bug (as well as a few of the fixes in https://github.com/contiki-os/contiki/pull/257 and https://github.com/contiki-os/contiki/pull/176).

The regression tests need https://github.com/contiki-os/contiki/pull/306 to be pulled in, as it uses the exp5438 platform and its 20-bit memory.