add password reset feature

How to test:

1. login, and visit http://gf2-roba.appspot.com/a/send_reset_password_email?email=your_email_address
2. You will receive an email, containing a random generated key
3. set new password by http://gf2-roba.appspot.com/a/a/reset_password?email=your_email-address&password=new_password&key=received_key