Added support for ID Site Logout

I personally like this approach because the user explicitly says that the URL that is building is to the "logout endpoint" instead of the regular '/sso' endpoint. Small suggestion, I think `forLogout()` reads better for the purpose of the user indicating that the URL is being built to that endpoint.

I think in the JavaDoc we should explain a little better what this is about, a user that has a session open to the application wants to logout from it, to execute this operation the application needs to create a signed request to Stormpath to the logout endpoint i.e. /sso/logout, if successful the user will be redirected to the callbackUri with the assertion of the account that logout. This is not my suggested wording, just a high level explanation of what will happen in order to document better.

Finally, if we're OK with this approach, the javadoc should indicate that once they call this method, there is no way to create a url that points to the regular sso endoint.

Any thoughts @lhazlewood ?