Add authentication acceptance test.

Before coding on the Authentication functionality of gophercloud, we
needed the acceptance test to know when we were finished.

Since the authentication test is written in Go, I had to adjust the
scripts/test-all.sh file to invoke Go programs directly.
diff --git a/acceptance/01-authentication.go b/acceptance/01-authentication.go
new file mode 100644
index 0000000..643ad48
--- /dev/null
+++ b/acceptance/01-authentication.go
@@ -0,0 +1,32 @@
+package main
+
+import (
+	"os"
+	"fmt"
+	"github.com/rackspace/gophercloud"
+)
+
+func main() {
+	provider := os.Getenv("SDK_PROVIDER")
+	username := os.Getenv("SDK_USERNAME")
+	password := os.Getenv("SDK_PASSWORD")
+
+	if (provider == "") || (username == "") || (password == "") {
+		fmt.Fprintf(os.Stderr, "One or more of the following environment variables aren't set:\n")
+		fmt.Fprintf(os.Stderr, "  SDK_PROVIDER=\"%s\"\n", provider)
+		fmt.Fprintf(os.Stderr, "  SDK_USERNAME=\"%s\"\n", username)
+		fmt.Fprintf(os.Stderr, "  SDK_PASSWORD=\"%s\"\n", password)
+		os.Exit(1)
+	}
+
+	_, err := gophercloud.Authenticate(
+		provider,
+		gophercloud.AuthOptions{
+			Username: username,
+			Password: password,
+		},
+	)
+	if err != nil {
+		panic(err)
+	}
+}