Refactor image_url method.

* Passing `options` to the image_url method didn't make sense since
`options` is an instance method.

* Now the case where an url does not end in `photo.jpg` is also handled: the
original image is returned instead. A test case was added for this situation.
Correct README.

As per the OAuth 2 docs [1], you only need `:prompt => 'consent'` to force
re-authorization and receive a refresh token.

[1]: https://developers.google.com/accounts/docs/OAuth2Login#offlineaccess