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`**

handlers/new_session.go

@@ -9,6 +9,13 @@ import (
 )
 
 func NewSession(rw http.ResponseWriter, req *http.Request) {
+	if !services.IsHuman(req) {
+		// User it not a human
+		rw.WriteHeader(http.StatusConflict)
+		rw.Write([]byte("Only humans are allowed!"))
+		return
+	}
+
 	s, err := services.NewSession()
 	if err != nil {
 		log.Println(err)