Usage:

• Browse to /app_dev.php
• When you see the user/password box, you can use admin:kitten or ryan:ryanpass credentials to login.
• Clear browser (like session cookies)
• Browse to /app_dev.php?auto_login=cnlhbjpyeWFucGFzcw==
• This should automatically log you in as user ryan without dialog box

Note: All the fun stuff is located in the src/Rainbow/SecurityBundle/Firewall directory