Added google authentication feature

Keys stored at setting are configured for localhost.
To create a new app, use google dev console:

1. https://code.google.com/apis/console/b/0/?noredirect
on the left panel, create new app (use old style console)

2. Choose it, select "API settings", Create OAuth2.0 client ID (https://www.dropbox.com/s/jbo8pqwvsbxlw5u/Screenshot%202014-06-05%2018.59.13.png)
https://www.dropbox.com/s/59crh8gs7zxonpl/Screenshot%202014-06-05%2018.59.52.png

3. Copy ClientID, secret
Set Redirect URI to `<yourhost>/auth/google_oauth2/callback`
and javascript origins : `<yourhost>`

Then update Settings accordingly

NB: auth should be added in my settings screen in order for the user to be able to login with it. 
New user won't be created based on google auth because there is no signup process implemented, so I decided not to change it.