Add maintenance logic to nginx config

- show a maintenance page iff the shared/system/maintenance file exists
- do not return our nginx version in headers
- show a 500 error page in the case of 502 (unicorn not running) errors.
Add cap tasks to show/hide the maintenance page

- cap <area> und:show_maintenance
- cap <area> und:hide_maintenance
Merge pull request #88 from ndlib/nginx-maintenance

Nginx maintenance