Update main.py

Bug fix (although untested it should work).

If one tries to verify an account that has already been verified, signup_token will no longer exist, thus user will be none and webapp2.auth.set_session will fail (IMO this is a bug with webapp2 trying to access user['user_id'] when user is None)
Merge pull request #3 from robert-king/patch-1

bug fix in verification handler