Add 2-Factor Authentication

[tjhorner]

I'd like to add these forms of 2FA:

• Authy/Google Authenticator time-based authentication
• Security Key (like YubiKeys)

In the future, it would also be nice to have 2FA over SMS, but that's not very necessary.

[tjhorner]

Added Authy/GA authentication in 6a3a42f.

The TwoFactorMethod model was made to support multiple Two-Factor authenticators, such as an Authy authenticator and YubiKey authenticator. The 2FA validation method on the User object iterates through all of these and returns true if at least one matches. This'll make it pretty easy to add a YubiKey method, I'll just need to add a type field.

[tjhorner]

I'll consider this done, and open an issue later for YubiKey 2FA.