Add support for URL prefixes

Usage example:


    handler := rest.ResourceHandler{}
    handler.SetRoutes(
        rest.Route{"GET"		,"/countries"		,GetAllCountries},
        rest.Route{"POST"		,"/countries"		,PostCountry},
        rest.Route{"GET"		,"/countries/:code"	,GetCountry},
        rest.Route{"DELETE"		,"/countries/:code"	,DeleteCountry},
    )
    handler.Handle("/api/")
    http.ListenAndServe(":12345", nil)