Add public SignOut method to CasAuthentication class

[rdev5]

By default, Request Validation in ASP.NET kicks in upon receiving a single sign-out request from CAS due to the nature of its POSTed content (i.e. XML).

This pull requests provides utility for developers to call CasAuthentication.SignOut(); which will not only invoke FormsAuthentication.SignOut (as a native replacement to ClearAuthCookies()) but also clear the user's service ticket from cache, which is currently not being done.

[serac]

The documentation you cited mentions HTML content specifically, but the payload of the single sign-out request is XML. Is XML specifically considered "potentially dangerous content," thus causing requests to be rejected? Just trying to follow your logic on the problem here.