Add a prefix property to the Request class

The prefix property contains the URL part after the first slash.
If there is no trailing slash, prefix equals to "/"

Prefix for http://www.foo.com/ = "/".
Prefix for http://www.foo.com/page = "/"
Prefix for http://www.foo.com/page/ =  "page"
Prefix for http://www.foo.com/page/10/ = "page"