Restore constructor which was removed

Restore constructor which breaks OauthWebSecurity
This is to fix issue 251 on Github:
https://github.com/DotNetOpenAuth/DotNetOpenAuth/issues/251

This will re-break the following issue, which needs more thought to
correctly implement: 

http://stackoverflow.com/questions/12235395/openauth-requestauthentication-throws-null-ref-in-vs2012-web-forms-template/12240150#12240150