Add avx2

@mortenvp @jpihl I updated this branch with the new dispatcher, and the previous valgrind issues are gone :)))
We still have this Macports g++ problem that we will not be able to fix:
http://buildbot.steinwurf.dk:12344/builders/fifi-mac109_64-cxx_gxx47_x64-py27_x64/builds/84/steps/build/logs/stdio
I would say that we should disable g++ on Mac. Any objections?

There is another caveat related to the performance: After adding the avx2 layers, I measured a small slowdown (~3%) on the iPad2 (compared to yesterday's results with the new dispatcher). My assumption was that adding a new layer should not matter now, since the dispatching is performed only once when a stack is constructed. We don't time the stack construction in the benchmark. What could go wrong here?