Use url builder instead of hardcoded urls

Hello,

First of all I would like to give big thanks for everyone who was involved in this good project.

We've failed to setup circus-web project in not-rooted dir behind proxy using following configuration:

http://localhost:80/**circus/**, Nginx as proxy
http://localhost:8080/, circushttpd as upstream web server

In order to solve this issue I did a switch from hardcoded urls and str.format urls into url builder provided by bottle framework.

Please consider accept those changes. Hoping that they could be useful for someone else.

I'm going to carefully test everything during this/next week, so current pull request does not have rock solid quality.

I just want to share my intentions with you.

Thank you in advance.