Add support for setting forceAuth, comparisonType, bindingUri to the Saml2Client

The Saml2Client assumes default values for forceAuth (false), comparisonType and bindingUri (HTTP-POST). This patch allows the applications to customize these options.
Like Jérôme; I really appreciate the tests :+1: 
I think however that the most important test is not present: We must add a new class TestRedirectSaml2Client that extends TestClient and runs a complete authentication flow on TestShib with the redirect binding. We could by the way rename the previous class in TestPostSaml2Client.