Validates that user is a human.

Add google recaptcha as an initial page before creating any session.
To configure recaptcha there are 2 environment variables that are needed
`GOOGLE_RECAPTCHA_SITE_KEY` and `GOOGLE_RECAPTCHA_SITE_SECRET`.
The code contains development defaults that should be set in production
to real values.
**NOTICE: Development defaults assume that the domain is `localhost`**

templates/welcome.go

@@ -0,0 +1,21 @@
+package templates
+
+import (
+	"bytes"
+	"html/template"
+
+	"github.com/franela/play-with-docker/services"
+)
+
+func GetWelcomeTemplate() ([]byte, error) {
+	welcomeTemplate, tplErr := template.New("welcome").ParseFiles("www/welcome.html")
+	if tplErr != nil {
+		return nil, tplErr
+	}
+	var b bytes.Buffer
+	tplExecuteErr := welcomeTemplate.ExecuteTemplate(&b, "GOOGLE_RECAPTCHA_SITE_KEY", services.GetGoogleRecaptchaSiteKey())
+	if tplExecuteErr != nil {
+		return nil, tplExecuteErr
+	}
+	return b.Bytes(), nil
+}