Improve usage under configs with no auth

[nylen]

If we don't specify authentication settings in a configuration here is what happens after clicking the Sign In button:

WP.com

WP core (without OAuth1 plugin installed)

WP core (with OAuth1 plugin installed)

Next steps

• Maybe add a new auth provider auth/none.js and use it if the required parameters are not present - it doesn't make sense to have requests go through oauth1.js or oauth2.js if we know there is no auth.
• Detect this condition and make some UI changes accordingly (disable the Sign In button; maybe show an "unauthed" indicator instead).
• Add a note to the readme about this.
• Improve error handling for authentication requests in general.