Add "http-timeout" CLI flag.

This allows for an override of the default HTTP timeout for library HTTP requests.
Fixes #207.

cli.go

@@ -146,6 +146,10 @@ func main() {
 			Name:  "dns",
 			Usage: "Solve a DNS challenge using the specified provider. Disables all other challenges. Run 'lego dnshelp' for help on usage.",
 		},
+		cli.IntFlag{
+			Name:  "http-timeout",
+			Usage: "Set the HTTP timeout value to a specific value in seconds. The default is 10 seconds.",
+		},
 	}
 
 	err = app.Run(os.Args)

cli_handlers.go

@@ -32,6 +32,11 @@ func checkFolder(path string) error {
 }
 
 func setup(c *cli.Context) (*Configuration, *Account, *acme.Client) {
+
+	if c.GlobalIsSet("http-timeout") {
+		acme.HTTPTimeout = time.Duration(c.GlobalInt("http-timeout")) * time.Second
+	}
+
 	err := checkFolder(c.GlobalString("path"))
 	if err != nil {
 		logger().Fatalf("Could not check/create path: %s", err.Error())