Adds ability to automatically connect your Clef account

A common user scenario is where a user is trying to log in with Clef, but hasn't connected their Clef account to their WordPress account yet.

This PR handles this case better. Now, when a user tries to log in with Clef, if there's no WordPress account connected to their Clef ID, they will see the following message.

![screenshot 2014-07-10 20 46 25](https://cloud.githubusercontent.com/assets/1097953/3548439/1a54dbdc-08ae-11e4-88fa-7b3e90fb6822.png)

When the user logs in with their username and password, their Clef account will be automatically connected.

![screenshot 2014-07-10 20 46 31](https://cloud.githubusercontent.com/assets/1097953/3548445/29790f66-08ae-11e4-952e-f178118f347d.png)

And they'll be displayed a nice message. Woo!