UrlFor implemented

Hi! One common pattern I've seen in multiple web frameworks is the ability to assign a name to a route and generate the url for that route through an UrlFor method, which I think it's very useful. I've noticed Martini didn't have it so here's my attempt to implement it.

All tests are passing.

Some examples:
```go
// Naming a route
m.Get("/foo", func() {
    // Here be code
}).Name("my_fancy_route")

// Generating the url for your route
fmt.Println(m.UrlFor("my_fancy_route", nil))
// Outputs /foo

// Naming a route with params
m.Get("/foo/:bar/:baz", func() {
    // Here be code
}).Name("my_fancy_route_with_params")

// Generating the url for your route with params
fmt.Println(m.UrlFor("my_fancy_route_with_params", "param1", "param2"))
// Outputs /foo/param1/param2
```