Convert StaticOptions.Expires to func reference

Typically you want the `Expires` header to be dynamic e.g now + 1 week.

This PR allows the user to define a function for generating the header string e.g.
```Go
func ExpiresHeader() string {
        return time.Now().Add(time.Hour * 168).UTC().Format("Mon, Jan 2 2006 15:04:05 GMT")
}

m.Use(martini.Static("public", martini.StaticOptions{Expires: ExpiresHeader}))
```