Force users to choose an account.

This forces users to choose the account they login into. This prevents accidental logins and the hassle of logging out your entire google session. 

Now users will get this screen - even if they are only logged into one google account at the time. 

![sign_in_-_google_accounts](https://cloud.githubusercontent.com/assets/882381/4535285/cfc66e30-4db0-11e4-9b62-287a01c75eee.png)

This (and  #243) should solve #234