Implement two-factor authentication support

Sending this feature as a pull request and will merge it in a couple of days after some additional testing and review.

This code adds new screen to enter two-factor authentication OTP code. 
Works with both ```application``` and ```sms``` authentication types.
In case when 2FA is not enabled for a user, auth flow remains as before.

Tested on my Galaxy Nexus with Android 4.3.

Refs #404