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
			},
		},
	})
}