Direct Authorization with GitHub

An even lower barrier-to-entry. Running `liferaft auth` without specifying a token will prompt for a GitHub username & password.

If Two-Factor Authentication is enabled, the user is further prompted for this code.

A "Laravel Liferaft" authorization is automatically created if one does not already exist, and the token is saved.

![image](https://cloud.githubusercontent.com/assets/510740/4244473/d573dc5c-3a21-11e4-951a-714d6dda1ee9.png)