add optional profiling middleware on debug

This adds an optional profiling middleware which can be activated by adding `adhocracy.enable_profiling = True` in `etc/adhocracy.ini`.

I chose [werkzeug](http://werkzeug.pocoo.org/docs/contrib/profiler/) over [paster](http://pythonpaste.org/modules/debug.profile.html) because it offered more customization options and a more useful output. Note that this reverts #255.