Add structs.Names() method

[fatih]

Just to return the names of each struct field. It'll be handy to use in a for range:

type Server struct {
    Name     string
    Number   int
    Disabled bool
}

s := new(Server)
for _, name := range structs.Names(s) {
    // prints: "Name", "Number" and "Disabled"
    fmt.Println(name)
}