add alternative git revision endpoint option, spec, and documentation

So my web server is set to use sendfile in all subdirectories of /system in my Rails app. I can't use the default endpoint because nginx just returns a 404 since the file 'revision' does not exist. This allows you to set any arbitrary endpoint in your rackup file if needed. It defaults to '/system/revision' so no existing code will have to be changed. Includes change, test, and documentation update.