Allow configuring pumactl with config.rb, closes #331

example:
  echo "pidfile '/tmp/app.pid'; rackup 'config.ru'" > /etc/puma/app.rb
  pumactl -F /etc/puma/app.rb start # starts an app and store pid in /tmp/app.pid
  pumactl -F /etc/puma/app.rb stop  # stops the started earlier app

Integration test t2 included: rake test:integration
Also fixed discrepancy in naming options :pidfile/:pid_file and :state/:status_file

This PR closes #331