For some reason unknown to me there is no reboot option in web-based user interface but it doesn't mean that it cannot be done easily as you just need to use one specific URL parameter.
Using web browser
http://192.168.1.1/setup.cgi?todo=reboot and provide your login credentials.
$ wget -O /dev/null -nv --user admin --password 'password' http://192.168.1.1/setup.cgi?todo=reboot
$ curl -o /dev/null -s -u 'admin:password' http://192.168.1.1/setup.cgi?todo=reboot
28.12.2014 - Update
I was pointed out by Daniel Kämmerling that recent firmware version does not support this functionality anymore, but you can use the following version.