Fetcher HTTP: Add support for dynamic URL

[moisespsena]

Add support for dynamic URL with URLFunc Attribute:

func main() {
    overseer.Run(overseer.Config{
        Program: prog,
        Fetcher: &fetcher.HTTP{
            URLFunc: func() (URL string, err ERROR) {
                t := time.Now()
                nows := fmt.Printf(
                    "%d-%02d-%02d_%02d%02d%02d",
                    t.Year(), t.Month(), t.Day(),
                    t.Hour(), t.Minute(), t.Second(),
                )
                return "http://localhost:4000/binaries/myapp-" + nows, nil
            },
        },
    })
}