Support for absolute urls and config base_url in UrlBuilderFactory

I am currently not sure if we should pass the ```base_url``` in the constructor of UrlBuilderFactory or via ```getUrlBuilder```.
I added it to ```getUrlBuilder``` this would allow us to exchange or set the config later via setter, cause the factory doesn't know about the ```base_url```, just when we request a ```UrlBuilder```.

Closes #60.