Add 'restart' and 'console' rake tasks. Stop nginx gracefully

On my machine nginx workers are not killed when the master process is (I
think the wrong signal is sent). Fortunately nginx allows to stop a
running instance gracefully (see http://wiki.nginx.org/CommandLine#Stopping_or_Restarting_Nginx)

The 'restart' task just calls 'stop' and 'start' tasks and the 'console' task loads the environment and starts an IRB shell.