Support for both authentication and authorization process

The [Twitter documentation](https://dev.twitter.com/pages/sign_in_with_twitter) describes two ways of authenticating an account:

* using the authentication method: the user is asked to grant the application access to her account without checking whether this has already been done
* using the authorization method: the user is asked to grant the application access to her account once only (this is referred to as "sign-in with twitter")

This commit adds the "authenticate" methods to use the slightly different process flow supported by Twitter.

Up to now, the package unfortunately used the terminology in the wrong way - the auth_twitter_authenticate_url returned an authorization URI, but should instead return an authentication URI. Thus, I have changed the terminology of the existing method and added a second method called auth_twitter_authorize_url, which will of course return the authorization URI. Both methods are supported by the underlying Net::Twitter package.

Altough this commit is backwards compatible, it will change behaviour. Not sure whether this is okay or not - please tell me. I thought it would be better to get the terminology right.