Use a vendor directory for repeatable builds - fixes #816

This is using godep to manage the vendor directory.

vendor/github.com/skratchdot/open-golang/open/exec.go

@@ -0,0 +1,18 @@
+// +build !windows,!darwin
+
+package open
+
+import (
+	"os/exec"
+)
+
+// http://sources.debian.net/src/xdg-utils/1.1.0~rc1%2Bgit20111210-7.1/scripts/xdg-open/
+// http://sources.debian.net/src/xdg-utils/1.1.0~rc1%2Bgit20111210-7.1/scripts/xdg-mime/
+
+func open(input string) *exec.Cmd {
+	return exec.Command("xdg-open", input)
+}
+
+func openWith(input string, appName string) *exec.Cmd {
+	return exec.Command(appName, input)
+}