Add support for percent-encoded slashes in parameter values

[coditect]

Passing percent-encoded slashes in URL parameter values is a real, albeit uncommon, use case. This pull request leverages new features introduced in Go 1.5 to enable httprouter to differentiate between the literal slashes that delimit path segments in a URL from encoded slashes that should be treated as data.

Given a simple HTTP server like this:

package main

import (
    "fmt"
    "github.com/julienschmidt/httprouter"
    "net/http"
)

func main() {
    server := httprouter.New()
    server.GET("/version/:v", handle)
    http.ListenAndServe(":8080", server)
}

func handle(w http.ResponseWriter, r *http.Request, p httprouter.Params) {
    fmt.Fprintf(w, "Version is %s\n", p.ByName("v")
}

A GET request to /version/go%2f1.6 should produce a response like this:

HTTP/1.1 200 OK
Content-Length: 17
Content-Type: text/plain; charset=utf-8

Version is go/1.6