If system/maintenance.html exist, return 503

This is required for our maintenance task [1].

[1] https://github.com/capistrano/maintenance
Return 200 instead of 503

Since we have HAProxy in front of our Ruby
applications, we can't return 503. HAProxy
will disable the backend and show it's own
error message.
Remove extra newline
Cache favicon.ico and robots.txt for 1h
Bump Gem version to 1.3.0
Merge pull request #12 from twingly/nginx-maintenance

If system/maintenance.html exist, show it