Support signing in directly, without an email link

[sander]

Not sure whether we want this yet. There are some attractive options however:

GITkit
http://code.google.com/apis/identitytoolkit/
pro: works directly with some email providers, no need for additional passwords
con: for other providers, there would be either password entry or no direct sign-in

BrowserID
https://browserid.org/
pro: possible future integration with Firefox, universal
con: requires tedious account creation through a third party like Mozilla